Merged multi-auth branch to trunk. See the authentication docs for the...

Merged multi-auth branch to trunk.  See the authentication docs for the ramifications of this change.  Many, many thanks to Joseph Kocherhans for the hard work! 


git-svn-id: http://code.djangoproject.com/svn/django/trunk@3226 bcc190cf-cafb-0310-a4f2-bffc1f526a37