Deprecate several APIs before 0.5.0 release

[JoshRosen]

This commit deprecates several APIs in advance of the 0.5.0 release:

• Deprecate the Scala helper functions, since they don't save much typing. Unlike some of the other data sources where the typical configuration consists only of a filename (such as Avro or Parquet), Redshift has several required configuration parameters. These convenience methods are really only saving users from the burden of writing "com.databricks.spark.redshift", which doesn't seem like a huge win.
• Deprecate the variant of redshiftFile which accepts a schema string. This code path is not tested and is likely to diverge over time. Most users will be better off using the data sources API or performing the type casts themselves.
• Remove the documentation for the overwrite configuration option, which is only used by the deprecated APIs. For consistency / minimizing the testing surface, I think that the code paths for dealing with overwriting should be written with reference to data sources' SaveMode API. I'm going to add some additional unit tests for this in a separate followup PR to make sure that the SaveMode flags are handled properly.
• Remove the documentation for the other deprecated methods.

[codecov-io]

Current coverage is 88.38%

Merging #65 into master will decrease coverage by -0.36% as of d7080f4

@@            master     #65   diff @@
======================================
  Files           10      10       
  Stmts          391     396     +5
  Branches        93      95     +2
  Methods          0       0       
======================================
+ Hit            347     350     +3
  Partial          0       0       
- Missed          44      46     +2

Review entire Coverage Diff as of d7080f4

Powered by Codecov. Updated on successful CI builds.

[JoshRosen]

I'm going to merge this now.