Fixed #13956 -- Enabled `*args` and `**kwargs` support for `simple_tag`,...

Fixed #13956 -- Enabled `*args` and `**kwargs` support for `simple_tag`, `inclusion_tag` and `assignment_tag`. Many thanks to Stephen Burrows for the report and initial patch, to Gregor Müllegger for the initial tests, to SamBull for the suggestions, and to Jannis Leidel for the review and PEP8 cleanup.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16908 bcc190cf-cafb-0310-a4f2-bffc1f526a37