Return the last time a given model's items were updated. Returns the
        epoch if the items were never updated.

def get_last_update_of_model(self, model, **kwargs):
        """
        Return the last time a given model's items were updated. Returns the
        epoch if the items were never updated.
        """
        qs = self.get_for_model(model)
        if kwargs:
            qs = qs.filter(**kwargs)
        try:
            return qs.order_by('-timestamp')[0].timestamp
        except IndexError:
            return datetime.datetime.fromtimestamp(0)