rowid_column becomes more like the "order by column". 'default_ordering' flag sent...

rowid_column becomes more like the "order by column".  'default_ordering' flag sent to create_engine enables whether or not the rowid_column on a Table will be None or not.  mappers/relations will by default use the rowid_column for ordering if its not None, else theres no default ordering.
still should better define 'default_ordering'/'rowid_column' relationship since its a little kludgy.