Add support for use as a Django templating engine

[earthboundkid]

Django 1.8 has added the ability to use custom templating backends. (See https://django.readthedocs.org/en/latest/topics/templates.html#custom-backends for more info.) I wrote a quick adaptor to make pystache work as a Django templating engine.

Would something like this be a good addition to pystache? If it's not a good fit to put in this project directly, would you be interested in supporting a django-pystache repo?