Fix last insert ID not being returned with postgresql.

When using PostgreSQL, upon saving new entries id is not being updated. 

That's because [PDO's implementation](http://lv.php.net/manual/en/pdo.lastinsertid.php) of lastInsertId requires sequence name as a parameter. One of three available options is fetching LASTVAL(). All three are described in the answer to [this SO question](http://stackoverflow.com/questions/2944297/postgresql-function-for-last-inserted-id).

I 'fixed' it by adding driver specific last_insert_id() method to ORM class.