This fixes the extension cache clearing and other compatibility issues
hit with newer versions of Django. We were using some private APIs in
earlier versions of Django, which have been replaced (largely by proper
APIs) in newer versions of Django.

The old template loader has been marked deprecated, and will be removed
in a future release. The loader will now show a warning when in use.
Documentation has been added to cover this.

Related unit tests pass on Django 1.6 and 1.8.