Add postgres schema support

Hello,
I've been using knex for many projects and I see that it didn't have a good support for [postgres schemas](http://www.postgresql.org/docs/8.1/static/ddl-schemas.html#DDL-SCHEMAS-CREATE). So, I created this pull request adding a new method `.using` which allows to scope table names within a schema (and adding the schema to the metadata queries).

Since many databases allows the "qualified name" structure for table names, I decided to put it into the generic objects `builder` and `compiler`, since it does not broken any API.

I added a few tests also and it seems to run fine (I don't have oracle/mariadb to run integration tests :disappointed: ).

So, do you guys think it is good to merge?