Fix crash in [NSManagedObjectContext rebuildAllContexts]

_mainContextTokenRef
_storeContextTokenRef
_backgroundContextTokenRef

may be NULL if you haven't accessed

[NSManagedObjectContext mainContext]
[NSManagedObjectContext storeContext]
[NSManagedObjectContext backgroundContext]
Merge pull request #13 from miniak/bugfix

Fix crash in [NSManagedObjectContext rebuildAllContexts]