Support Django 1.4, 1.5 and 1.6

For that, tox support is needed to test with different environments (i.e. against Django-1.4 and Django-1.5).

The default session serializer switched to JSONSerializer in Django-1.6. Unfortunately, it can't serialize arbitrary objects (such as datetime instances), there we have to stay with PickleSerializer (compare with 
https://docs.djangoproject.com/en/1.6/topics/http/sessions/#write-your-own-serializer).

Results: http://paste.opensuse.org/57865169