Add support for checking existence of labels

I'd like to check that a progress spinner is appearing and subsequently disappears as I'm transitioning through screen states. At first, I didn't realize that I could just use SLElement to match any element type, as it doesn't match the way I would use UIAutomation directly. Even still, I think it makes sense to have the type checking in 'matchesObject'. Thoughts?

Marking this as WIP, as I really should add an integration test before merging.
What about OR'ing the two conditions? That way if someone happened to subclass UILabel and made it non-static, that would still work as well. Granted, it would be a bit weird for someone to do...