Support custom field types (#342)

* Support custom field types

* Allow custom column type when defining a field

Require the user to provide the type that should be read from the RS for each converter
Add specs for each converter/driver

* Support up to i64 for enums

Add spec for to/from for each type

* Doc pass

* Add more readme headings

docs/models.md

@@ -0,0 +1,167 @@
+# Model Usage
+
+## Multiple Adapters
+
+It is possible to register multiple adapters with different names/URLs, for example:
+
+```Crystal
+Granite::Adapters << Granite::Adapter::Mysql.new({name: "legacy_db", url: "LEGACY_DB_URL"})
+Granite::Adapters << Granite::Adapter::Pg.new({name: "new_db", url: "NEW_DB_URL"})
+
+class Foo < Granite::Base
+  adapter legacy_db
+  
+  # model fields
+end
+
+class Bar < Granite::Base
+  adapter new_db
+  
+  # model fields
+end
+```
+
+In this example, model `Foo`, is connecting to a MySQL DB at the URL of `LEGACY_DB_URL` while model `Bar` is connecting to a Postgres DB at the URL `NEW_DB_URL`.  The adapter name given in the model maps to the name of a registered adapter. 
+
+> **NOTE:** How you store/supply each adapter's URL is up to you; Granite only cares that it gets registered via `Granite::Adapters << adapter_object`.
+
+## timestamps
+
+The `timestamps` macro defines `created_at` and `updated_at` field for you.
+
+```crystal
+class Bar < Granite::Base
+  adapter mysql
+  # Other fields
+  timestamps
+end
+```
+
+Would be equivalent to:
+
+```crystal
+class Bar < Granite::Base
+  adapter mysql
+    
+  field created_at : Time
+  field updated_at : Time
+end
+```
+
+## Custom Primary Key
+
+For legacy database mappings, you may already have a table and the primary key is not named `id` or `Int64`.
+
+Use the `primary` macro to define your own primary key
+
+```crystal
+class Site < Granite::Base
+  adapter mysql
+  primary custom_id : Int32
+  field name : String
+end
+```
+
+This will override the default primary key of `id : Int64`.
+
+## Natural Keys
+
+For natural keys, you can set `auto: false` option to disable auto increment.
+
+```crystal
+class Site < Granite::Base
+  adapter mysql
+  primary code : String, auto: false
+  field name : String
+end
+```
+
+## UUIDs
+
+For databases that utilize UUIDs as the primary key, the type of the primary key can be set to `UUID`.  This will generate a secure UUID when the model is saved.
+
+```crystal
+class Book < Granite::Base
+  adapter mysql
+  primary isbn : UUID
+  field name : String
+end
+
+book = Book.new
+book.name = "Moby Dick"
+book.isbn # => nil
+book.save
+book.isbn # => RFC4122 V4 UUID string
+```
+## Default values
+
+A default value can be assigned to a field that will be used if another value is not specified/supplies.
+
+```Crystal
+class Book < Granite::Base
+  adapter mysql
+  
+  field name : String, default: "DefaultBook"
+end
+
+book = Book.new
+book.name # => "DefaultBook"
+```
+
+
+## Generating Documentation
+
+By default, running `crystal docs` will **not** include Granite methods, constants, and properties.  To include these, have an ENV variable: `DISABLE_GRANITE_DOCS=false` set before running `crystal docs`.
+
+The `field` and `primary` macros have a comment option that will specify the documentation comment to apply to that property's getter and setter.
+
+`field age : Int32, comment: "# Number of seconds since the post was posted"`
+
+## Third-Party Annotations
+
+Annotations can be a powerful method of adding property specific features with minimal amounts of code.  Since Granite utilizes the `property` keyword for its fields, annotations are able to be applied easily.  This is accomplished by using the `annotations` option on a field, similar to the `comment` option above.  It is used in the form of `annotations: ["annotation1", "annotation2"]`, for example:
+
+```Crystal
+class Foo < Granite::Base
+  adapter mysql
+  table_name foos
+
+  field name : String
+  field password : String, annotations: ["@[Foo::Options(option1: true)]", "@[Bar::Settings(other_option: 7)]"]
+  field age : Int32
+end
+```
+
+Notice the values of the array are exactly as what you would put on a normal property, just as strings.  In this case, Granite will apply the two annotations to the `password` property only.
+
+## Converters
+
+Granite supports custom/special types via converters.  Converters will convert the type into something the database can store when saving the model, and will convert the returned database value into that type on read.
+
+Each converter has a `T` generic argument that tells the converter what type should be read from the `DB::ResultSet`.  For example, if you wanted to use the `UUID` converter and your underlying database column is `BLOB`, you would use `Bytes`, if it was `TEXT`, you would use `String`.
+
+Currently Granite supports various converters, each with their own supported database column types:
+
+- `Uuid(T)` - Converts a `UUID` to/from a database column of type `T`.  Supported types for `T` are: `String`, and `Bytes`.
+- `Enum(E, T)` - Converts an Enum of type `E` to/from a database column of type `T`. Supported types for `T` are: `Number`, `String`, and `Bytes`.
+- `Json(M, T)` - Converters an `Object` of type `M` to/from a database column of type `T.`  Supported types for `T` are: `String`, `JSON::Any`, and `Bytes`.
+  - **NOTE:**  `M` must implement `#to_json` and `.from_json` methods.
+- `PgNumeric` - Converts a `PG::Numeric` value to a `Float64` on read.
+
+The converter is defined on a per field basis.  This example has an `OrderStatus` enum typed field.  When saved, the enum value would be converted to a string to be stored in the DB.  Then, when read, the string would be used to parse a new instance of `OrderStatus`.
+
+```crystal
+enum OrderStatus
+  Active
+  Expired
+  Completed
+end
+
+class Order < Granite::Base
+  adapter mysql
+  table_name foos
+
+  # Other fields
+  field status : OrderStatus, converter: Granite::Converters::Enum(OrderStatus, String) 
+end
+```