implement Ecto.Adapters.Mysql.SQL based on Postgres version

@jquadrin  I had started this a while ago. This implements `Ecto.Adapters.Mysql.SQL` and `Ecto.Adapters.Mysql.SQLTest`. I've modified the tests to be MySQL specific and then updated the `Ecto.Adapters.Mysql.SQL` to get the tests to pass.