Complete sqlite3 dialect

[eaigner]

Implementing the migration system changed the dialect interface radically. Most methods have been ported from the postgres dialect, but sqlite3 only supports a subset of ALTER TABLE, so some methods were left out for time reasons.

Complete RenameColumn, ChangeColumn and DropColumn implementations.

[eaigner]

Note: https://github.com/rails/rails/blob/master/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb

[eaigner]

Since I currently don't have any need for Sqlite3, I most likely will not implement it myself, since Sqlite3 requires some special workarounds to emulate some advanced database functions.