Python 3 support

Support for python 3. Tested with python 2.5, 2.6, 2.7, 3.2, 3.3 (3.3 on my local machine, because travis does not support it yet), django 1.3, 1.4, 1.5.

Many apps depend on django-appconf (it even made it to Python 3 wall of shame - http://python3wos.appspot.com/), so it should support python 3 as fast as possible - to allow other apps migrating to python 3.