Adding python3 support

I'm trying to port Spirit to python 3, as it is a dependencie of it I need to port `django-djconfig`. The method used is the one preconised from [django doc](https://docs.djangoproject.com/en/dev/topics/python3/). 

The travis build is OK for **django 1.5, 1.6, 1.7** and **python 2.7, 3.3, 3.4** see <https://travis-ci.org/benjaminabel/django-djconfig/builds/39861832>.