Added support for pg8000 database adapter (and others)

This release adds support for database adapters other than psycopg2 (http://initd.org/psycopg/); specifically, it adds support for pg8000 (https://github.com/mfenniak/pg8000/).

While this hasn't been tested with any other adapters (or, indeed, any other databases), it should provide good ground for adding new ones in the future.