Switch from Jinja2/Jingo back to Django templates

Originally, I switched from Django to Jinja2 templates back in the
Django 1.3 days. Since then, Django templates have added features
that make them easier for me to deal with and there really aren't enough
benefits for staying with Jinja anymore for this project.

Ergo, switching back to Django.

It's likely that there are still bugs, but they should get ironed out
before a release.

Additionally, while going through all the files, I discovered a few
lacked file headers. I added those.

Fixes #189.