Remove object from backing context if it has been removed from Parse

[sbonami]

Objects may be removed from other clients, therefore PFIS should remove objects from the local backing store if it cannot be found during a subsequent fetch. Otherwise, inconsistencies ensue and we have a client out of sync with Parse.

A few things need to be kept in mind:

• Some queries are allowed to return no objects, so only delete objects in instances where they have actually been removed.
• Make sure we don't delete objects that have been returned and are in the midst of being displayed to/used by the user; this would be BAD. Potential remedy: send an NSNotification if an object gets deleted after it has been fetched)