Document how to add a DTL template engine for 1.8 sites

[Osmose]

If a site is using Django 1.8 and does not have any TEMPLATES engine definition for Django templates (if they're using Jinja or something else instead), they'll need to add an extra engine definition specifically for our library if they want to use our helpers. We should document something to that effect.

(I have yet to actually get around to testing this, so I might be getting the details of the problem wrong.)

[Osmose]

Alternatively, maybe we can just provide Jinja2 templates and wait until someone complains that we don't support their custom template syntax.

[abompard]

The Django 1.8 documentation is not very clear yet on how libraries should handle custom template engines. In the meantime, the code in git now works with the DTL, and the currently released code does not work with Django 1.8 at all.
Would it be possible to cut a release with what we have now? My webapp uses django-browserid and I currently have to force users to use Django 1.7.

[Osmose]

I'm hoping to cut a release after #289 gets merged (I wrote it in response to one of my projects using Django 1.8 and django-jinja and failing when I updated django-browserid), and then land some documentation updates.

[Osmose]

@abompard FYI 1.0.0 is released with the Django 1.8 fixes. https://github.com/mozilla/django-browserid/releases/tag/v1.0.0