Added support for explicitly selecting an engine.
Removed private API get_template_from_string.
Removed extraneous arguments in Engine.get_template_from_string.

This aligns the Django Template Engine API with the common template
backend API.
Passed a reference to the current engine when instanciating Template.
Renamed get_template_from_string to from_string.

The shorter name is just as explicit and, well, shorter.