…ror on partials not found

…precompilation support

It is organized as follows:
* In anticipation of Django 1.8 template engine support, it creates a new file template/backends.py
  with a Handlebars engine structured for the Django 1.8 api (though missing some parts for full support)
* In backends.py there is also a CompiledTemplate which is subclassed from template.base.HandlebarsTemplate
* HandlbarsTemplate and BaseHandlebarsLoader are tweaked to leverage the code in backends.Handlebars.
* loader.reset_loaders is a new method, which helps testing
* loaders.filesystem.CachedLoader is mostly separate, but also helps keep template compilations in cache and reusable for future requests.

This is meant to gesture towards the following goals:
* maintaining backwards compatibility
* moving towards support for Django 1.8
* allowing djangobars to be used in a multi-threaded environment

Originally, I separated the thread safety and precompilation as projects, but in
production, we found that locking threads, still blocks too many requests, so
they are really necessary combined, at least, in practice.