Update to Django 1.7, propose SQLite usage

This PR does two things which depend on each other:

* Update Django to 1.7-RC1 - in my experience this is already quite stable. Furthermore south with SQLite doesn't work with Python 3, thus in order to use SQLite and Python 3, Django-1.7 is the way to go.
* Propose to use SQLite instead of PostgreSQL in development environments, as this decreases setup effort.