Support for NamedTupleConnection/DictConnection.

This allows using connection_factory types psycopg2.extras.NamedTupleConnection or DictConnection. If cursor_factory is not defined will be used connection_factory default.

Example:
self.db = momoko.Pool(dsn=settings['dsn'], size=settings['db_conn_size'], connection_factory=psycopg2.extras.NamedTupleConnection)