Imported BaseEngine from the DEP.

i18n is left aside for now.
Added initial support for loading template engines.
Added dummy template backend.
Added jinja2 template backend.
Added Django template backend.
Added basic tests for template backends.
Removed some uses of global APIs from django.template.loader.
Passed a reference to the current engine when instantiating Template.
Isolated template tests from Django settings.
Looked up the default template engine in the list of all engines.
Supported multiple template engines in get_template and select_template.

This commit changes the return type of these two functions. Instead of
returning a django.template.Template they return a backend-specific
Template class that must implement render(self, context).
Removed private API find_template.

It wasn't documented and it wasn't used anywhere, except in a few tests
that don't test it specifically and can be rewritten with get_template.
Removed private API get_template_from_string.

It wasn't documented and it wasn't used anywhere.
Removed extraneous arguments in Engine.from_string.

This aligns the Django Template Engine API with the common template
backend API.
Supported multiple template engines in render_to_string.

Adjusted its API through a deprecation path according to the DEP.
Added tests for django.template.loader.

Deprecated features aren't tested.
Added a comment about the last use of Engine.get_default().
Moved context_processors from django.core to django.template.
Simplified implementation of django.shortcuts.render(_to_response).

*args, **kwargs brought more confusion than concision.
Deprecated some arguments of django.shortcuts.render(_to_response).

dictionary and context_instance and superseded by context.

Refactored tests that relied context_instance with more modern idioms.