Provide python 3 compatibility

The codebase is now compatible with python2.7 and python3.3
I used the customized version of six bundled with Django as of version 1.4.2.

The tests are still ok for python2.7 + django 1.5.5.
**But 3 are failing in python3.3**

I will try to investigate, but explore it yourself, you should be faster than me !

There may be a final step in adding ``from __future__ import unicode_literals`` for better compatibility.