Add alias for Sequel::Postgres::Adapter to PG::Connection

Need to see how the various Mysql extension are handled - perhaps this can be done a better way.

I wasn't sure if I should add `'Sequel::MySQL::Adapter' => 'Mysql2::Client'` or not, as I'm not sure how everything's handled with MySQL.

Sequel tries to expose a consistent interface no matter which client you actually use (ie - `Sequel::Postgres::Adapter` actually extends `::PGConn` but tries to normalize the interface between the `pg`, `postgres`, and `postgres-pr` drivers.  However, I'm not sure how that's handled on the MySQL side and what would be best to expose.