* Converted the ``libraries`` and ``builtins`` globals of
``django.template.base`` into properties of the Engine class.
* Added a public API for explicit registration of libraries and builtins.
Since this package is going to hold both the implementation of the Django
Template Language and the infrastructure for Multiple Template Engines,
it should be untied from the DTL as much as possible within our
backwards-compatibility policy.
Only public APIs (i.e. APIs mentioned in the documentation) were left.
