adapted migrations to django 1.7

In order to use the django migrations included in django 1.7 django-oauth2-provider needs to be updated

I just followed the docs:
https://docs.djangoproject.com/en/1.7/topics/migrations/#libraries-third-party-apps

Basically move migrations directory to south_migrations and run makemigrations.

This solves the issue of referencing django-oauth2-provider models from a new django 1.7 migration. The historical cannot be retrieved properly.