Keep History - test post save handler

Add an option to GenericModerator that controls whether history of
 moderated objects should be kept or not.

Add a new "date_updated" field to ModeratedObject so that we can
 determine what the latest object is.

Update ModerationObjectManager.get_for_instance to try and get the
 moderated object the old way,.and if there are too many (because keep_history = True),
 find the most recent one and return that.

Add a test to make sure the post save handler works with keep_history

Add a test for ModeratedObjectManager to test get_for_instance