django 1.4 USE_TZ compat

[dwheaton]

Ran into trouble running django 1.4 with USE_TZ=True

File "...\persistent_messages\storage.py" in _message_queryset

1.     qs = Message.objects.filter(user=get_user(self.request)).filter(Q(expires=None) | Q(expires__gt=datetime.datetime.now()))
   

Simple enough fix so as time permits I will test a backwards-compatible fix.