Add support for PostgreSQL CREATE/DROP EXTENSION

This adds [`CREATE EXTENSION`](http://www.postgresql.org/docs/9.3/static/sql-createextension.html) and [`DROP EXTENSION`](http://www.postgresql.org/docs/9.3/static/sql-dropextension.html) support for the `PostgreSQL` adapter: `#create_extension` and `#drop_extension`

    DB.create_extension(:hstore, :if_not_exists => true)
    # CREATE EXTENSION IF NOT EXISTS hstore

    DB.drop_extension(:hstore)
    # DROP EXTENSION hstore

- [x] Try to include tests for all new features.
- [x] Try to include documentation for all new features.
- [x] Follow the style conventions of the surrounding code.
- [x] Do not submit whitespace changes with code changes. (This actually happened, and I had to go back, search for the specific instances and add back the appropriate amount of trilling whitespace, because at the time, it was the easiest solution)
- [x] All code in pull requests is assummed to be MIT licensed.