Move invocation of 'setup_generic_relations' to an AppConfig instance to...

... guard against premature model access.

This pull works towards Django 1.7 compatibility. 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

Initialization code should be moved to an AppConfig instance on the ``ready`` method.