Added django debug toolbar configuration

By setting the corresponding environment variable, the django debug toolbar is loaded. This is very useful for debugging templates and SQL queries.

![1](http://tmp.dbrgn.ch/screenshots/20140405_143949.png)

![2](http://tmp.dbrgn.ch/screenshots/20140405_144011.png)

To enable, install `django-debug-toolbar` via pip and start the server like this:

    DJANGO_DEBUG_TOOLBAR=1 ./manage.py runserver