For ticket #20625 to allow defining chainable QuerySet on custom model managers.

Ticket #20625 - https://code.djangoproject.com/ticket/20625 allows for new functionality as follows: 

    class OfferManager(models.Manager):
        """ Example of a chainable custom query set """
        
        ...
        
        QUERYSET_PUBLIC_KWARGS = {'status__gte': STATUS_ENABLED}
        QUERYSET_ACTIVE_KWARGS = {'status': STATUS_ENABLED}
        
        ...
        
        def public(self):
            """ Returns all entries accessible through front end site"""
            return self.all().filter(...)
        public.chainable = True     # instructs to dynamically tranplat this method onto
                                    # returned QuerySet as <queryset>.public(...) 
                                    # effectively providing chainable custom QuerySets

        def active(self):
            """ Returns offers that are open to negotiation """
            return self.public().filter(**OfferManager.QUERYSET_ACTIVE_KWARGS)
                                        # an example of how to reffer to OfferManager
                                        # constants as 'self' context changes
        active.chainable = True
        ...