Bugfix/fetch request managed object cache no subentities

I was in need of a way to ignore sub-entities when looking for entities during mapping, and found I could do so by adding a configurable property to RKFetchRequestManagedObjectCache to disable them in the results of that class.

I thought this might be generally useful, have a look and see.

I'm new to the unit test setup, but I did add a unit test for this functionality, perhaps not in the right place.