Feature/deleting

Hi Mattt and others,
There are several issues already discussing the management of soft deletes coming in the server response. I couldn't find a good way to implement that in a subclass without modifying AFIncrementalStore, since the local store, backing store, and insertOrUpdateObjectsFromRepresentations:ofEntity:fromResponse:withContext:error:completionBlock: are all private.

So I took a stab at it. Feedback is welcome.