This adds two new template loaders intended to directly replace standard

Django template ones.
The new 'conditional_cached' template loader functions like Django's

'cached' loader, but does not cache if DEBUG is True. This helps a lot

with development, since it's no longer necessary to reload the server

after modifying a template.
The new 'namespaced_app_dirs' template loader is like Django's

'app_directories' loader, but accepts an optional namespace prefix in

the form of 'app.path:'. If specified, the list of template directories

will be limited to those owned by the given app path.

Tested both of these with Review Board. I didn't have caching issues in

development anymore, and was able to look up by namespace when extending

a template.