Improvements to get to make templates more reusable

Fix a detail [2d9bbd5], now works with 3 levels [94797dd] and tomorrow will add the changes for this feature works with the [cached template loader](https://github.com/django/django/blob/master/django/template/loaders/cached.py).

@unaizalakain you need a lot of test to this feature, you could copy this from [django-smart-extends repo] (https://github.com/goinnn/django-smart-extends/blob/master/example/example/app/tests.py), these should be adapted, but this is easy :-)