Custom serializer that emits raw CaseXML when ?format=xml is requested

Simple serializer that opts for `CommCareCase.to_xml('2.0')` when XML serialization is requested. A bit funky that this representation differs so greatly but hey.

Since the override hooks in Tastypie are not quite adequate I did have to reproduce some of their code here - the alternative was simply reimplementing which adds up to the same amount of trouble. Licenses are compatible.