Fixed #5786: relaxed the validation for usernames to allow more common characters '@', etc.

This is really just a stop-gap until we come up with a improved way of handling
disparate auth data, but it should help us stretch a bit more milage out of the
current system.

Thanks to alextreme, lbruno, and clayg.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12634 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/topics/auth.txt

@@ -71,6 +71,9 @@ Fields
 
         Required. 30 characters or fewer. Alphanumeric characters only
         (letters, digits and underscores).
+        
+        .. versionchanged:: 1.2
+           Usernames may now contain ``@``, ``+``, ``.`` and ``-`` characters.
 
     .. attribute:: models.User.first_name