[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.
I think that both points are actually general advice regarding `SLElement`. The documentation of `SLElement`'s constructors link to the [UIAccessibility Protocol Reference](https://developer.apple.com/library/ios/documentation/uikit/reference/UIAccessibility_Protocol/Introduction/Introduction.html) to provide general guidance around how labels might be derived or set.

Besides those notes, it might be worth adding a general "how to match elements" guide to the wiki somewhere.

The `accessibilityIdentifier` suggestion is definitely worth adding to `+[SLElement elementWithAccessibilityIdentifier:]` and/or the wiki.