Added macro to stub properties that work with sort descriptors, predicates etc

I ran into the following issue: when stubbing a property value using 
```objective-c
[given([obj someProperty]) willReturn:@42)];
``` 
it doesn't work with `NSSortDescriptor` or `NSPredicate` and some other API that relies on `valueForKey:` and `valueForKeyPath:` instead of direct querying the properties.

That's why I added a small macro `stubProperty` (or `MKTStubProperty`) to address this issue. Please let me know what do you think of this implementation.