Add method that counts matching objects in fetch request

It's probably a good idea to have a method that counts the matching objects of a fetchRequest. 
In theory you could use the fetch method that returns a NSArray and count yourself, but `countForFetchRequest:error:` will be much faster. 

Access to the substituted fetchRequest would be beneficial as well, especially if you create a NSFetchedResultsController or if you want to access the predicates defined in the model editor.