Add method to clear thread-local context

When running unit tests that are asynchronous/multi-threaded, I find that oftentimes a background thread will have an `NSManagedObjectContext` instance (created by `MR_contextForCurrentThread`) from a previous set up, even after calling `[MagicalRecord cleanUp]`

When this happens, I get the dreaded "Object's persistent store is not reachable from this NSManagedObjectContext's coordinator" error. Removing the MOC's from each thread's `threadDictionary` solves the problem.