[WIP] 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.
With something like the comment: "Such as, but not limited to, instances of `UILabel`." (See `SLButton`'s overview.)