Quick function to get cases referenced by an xform

Opening for discussion. Pertains to https://github.com/dimagi/commcare-hq/pull/409

Having gone through enough code to figure out how to do this and where, I sort of see why it does not exist already: It may not be specific enough? Perhaps just getting a list of "cases referenced by a form" is not a correct description of the use case for dereferencing in the API?