Revised package to not pull in breaking changes

@Suor: This looks like a cool library, thanks for building on top of sql-bricks!

I plan on making some breaking changes to the sql-bricks API in version 1.0 -- in particular, pulling the postgres-specific clauses (limit, offset, returning, and perhaps one or two more) into a separate extension file, so in order to get the core (SQL-92) functionality plus the postgres functionality, you would do require('sql-bricks/postgres') instead of require('sql-bricks').