Adds "All Data" filter to case list report

Addresses this ticket:
http://manage.dimagi.com/default.asp?121778#688842
It adds an "All Data" filter by modifying the `CaseListMixin` class that is subclassed by `CaseListReport`. I've examined the code of `CaseListReport`'s descendants, and none of them seem to cause conflicts with the changes made.
Thanks for the suggestions, `self.show_all_filter` will always be true if present. Making those changes now...