Fixed #6547, added support for GeoRSS feeds in `django.contrib.gis.feeds`;...

Fixed #6547, added support for GeoRSS feeds in `django.contrib.gis.feeds`; added the `feed_extra_kwargs` and `item_extra_kwargs` to the `Feed` baseclass so that it's possible for subclasses to add dynamic attributes.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8414 bcc190cf-cafb-0310-a4f2-bffc1f526a37