Support Django 1.7 in the iPython notebook extension

Django 1.7 introduced the app registry which must be initialized before we can call get_apps(). Normally Django does this as part of manage.py, but this isn't done when running as the iPython notebook extension isn't, so we need to check if the app registry is initialized and call django.setup() if it isn't.

I also removed the "temporary" workaround for 0.96 and older.