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)
~~~