Supported multiple template engines in render_to_string.

Adjusted its API through a deprecation path according to the DEP.
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dictionary and context_instance and superseded by context.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.
Preserved context class in inclusion_tag.

Previously, when a template was rendered with RequestContext, inclusion
tags were rendered with a plain context, losing additional information
available in the RequestContext.

The (admittedly bizarre) implementation of RequestContext.new() has the
side-effect of not running template context processors, making this
change backwards-compatible.
Deprecated current_app in TemplateResponse and render(_to_response).
Raised an error when allowed_include_roots is a string.

This avoids leaving projects silently vulnerable when this option is set
to a string instead of a one-item tuple containing that string, a very
common misconfiguration.
Deprecated ALLOWED_INCLUDE_ROOTS.
Deprecated TEMPLATE_STRING_IF_INVALID.
Deprecated TEMPLATE_LOADERS.
Deprecated TEMPLATE_DIRS.
Deprecated TEMPLATE_CONTEXT_PROCESSORS.
Raised a warning when using the legacy TEMPLATE_* settings.

All tests now rely on TEMPLATES.