Some API changes to SuperPage

1) Ditched get_subpage_urls() method. Developers should now use the subpage_urls attribute.
   I've made this change as I think the vast majority of use cases for SuperPage won't require dynamically updating URLs.
   This change will make caching of generated URL configs much easier to implement if we find we need it.

2) reverse_subpage() now has a similar signature to urlresolvers.reverse() and RouteResult()
   This should make it easier to remember how the args and kwargs should be passed into this method