CoreData to Parse NSPredicate translation

There should be some mechanism to translate between CoreData and Parse NSPredicates.
This PR i.e. allows to properly map between local and remote objects in fetch predicates:

```objective-c
    NSPredicate *predicate = [NSPredicate predicateWithFormat: @"relatedObject == %@", managedObject];
    fetchRequest.fetchPredicate = predicate;
```

This implementation could be improved, i.e. now it substitutes the managedObject with a PFObject that is being fetched on the main thread. Probably it would be better to build a native Parse NSPredicate instead of substituting the constant value in the NSExpression.