[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.
Also, what do you think about matching objects with the `UIAccessibilityTraitStaticText` trait instead of specifically `UILabel`, @justinseanmartin? That's matching more on behavior rather than implementation and would allow matching e.g. "labels" in web views too.