Configurable primary_key

[caspiano]

# usage
class Model < RethinkORM::Base
  primary_key hash
  attribute hash : String
end

This will require configuring the table with the desired primary_key on startup.
We'll also need a validator to ensure that key lengths are less than 127 chars (a RethinkDB system limit).

We'll also need to update queries to use the primary key by default.