Fixed #2066: session data can now be stored in the cache or on the filesystem. This should be fully backwards-compatible (the database cache store is still the default). A big thanks to John D'Agostino for the bulk of this code.

      git-svn-id: http://code.djangoproject.com/svn/django/trunk@6333 bcc190cf-cafb-0310-a4f2-bffc1f526a37