Make views.login compatible with django 1.4 by updating messaging

The Message model (in django.contrib.auth), its related manager in the User model (user.message_set), and the associated methods (user.message_set.create() and user.get_and_delete_messages()), which have been deprecated since the 1.2 release, have been removed from Django 1.4.

This PR updates the `views.login` call to use the messages framework instead of the Message model.

See https://docs.djangoproject.com/en/1.3/internals/deprecation/ and https://docs.djangoproject.com/en/1.3/ref/contrib/messages/