Python3 support

All tests works now in python 2.7 and 3.4. No test has been removed.

The method follows django project recommendations([Porting to python 3](https://docs.djangoproject.com/en/dev/topics/python3/))

- `unicode_literals` evreywhere
- changed the string representation of objects with the the `@python_2_unicode_compatible` decorator
- relative imports changed
- usage of `django.utils.six` methods when provided, or if if `six.PY2:` statement if not.
- some assert methods changed
- some string/bytes changes in json.loads(maybe we can use the django method provided in django 1.7), and in hashlib.

I'm gonna try tun the app manually now to see if everything works, and maybe add some more tests to the app if not.