Python 3 support

A few things to note about this pull request:
* It currently relies on singingwolfboy/XBlock instead of edx/XBlock. This can change once https://github.com/edx/XBlock/pull/205 is merged.
* [Django doesn't support Python 3 until at least version 1.5](https://docs.djangoproject.com/en/dev/topics/python3/), so I had to update the version of Django in the requirements file
* As a result of upgrading Django, I had to slightly change the syntax of the `{% url %}` calls in the Django templates.
* There are still a few failing tests, but the failures don't seem to be due to changes specific to Python 3. Perhaps someone can take a look?