Django 1.7 compatibility effort

This pull works towards Django 1.7 compatibility. Items listed below:

 * AppConfig instance for initialization code (setting up generic relations on actionable models)
  * Previously, model access at import-time (using ``get_model``) was safe. In 1.7, this raises a RunTime error when the app registry isn't ready. See: https://docs.djangoproject.com/en/dev/releases/1.7/#start-up-sequence
 * ``ReverseGenericRelatedObjectsDescriptor`` has moved in to a different module