Fixed #689 -- Added a middleware and authentication backend to contrib.auth for supporting external authentication solutions.  Thanks to all who contributed to this patch, including Ian Holsman, garthk, Koen Biermans, Marc Fargas, ekarulf, and Ramiro Morales.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10063 bcc190cf-cafb-0310-a4f2-bffc1f526a37