Support Django 1.7 as_manager

[streeter]

Django 1.7 introduces a new method to create managers with custom querysets. I think we should make sure that method works. That would mean moving more logic to the queryset from the manager class.

[frewsxcv]

Most of our logic is already on the queryset at this point. This isn't necessary to implement to support Django 1.7, but once we drop Django 1.6 support, we can remove the intermediate manager and use as_manager.