Python 3 support

[moggers87]

While working on PR #19 I found that django-session-csrf doesn't work under Python 3.

Probably not much work as this module is quite small.

[moggers87]

I have Python support in my fork: https://github.com/moggers87/django-csrf-session

If there's interest here, I can make a PR

[peterbe]

Please please please revive this.
We rely on django-session-csrf in sugardough and I just today discovered that django-session-csrf is the thing that breaks sugardough in python 3.

Make a PR (rebased up) and we can discuss it there.

[peterbe]

Let's get the basics working first of all. I.e. it running in Python 3.
Then we really should work on adding tox. And travis should run the tox script. That way local devs and travis tests python X compatibility the same way.