Add capturing screenshots to SLDevice

Hey I have to run but I saw Jeff was on so I thought I'd put this up (Not ready to merge -- no documentation).

1. Any ideas on how to test screenshot functionality? Haven't thought about this too hard but presumably difficult.
2. How to represent the `Rect`, `Point`, etc. objects in UIAutomation? For now I just made a function that takes a `CGRect` and turns it into the literal string used to make the `Rect` object. Probably at the least I should move this to like an `SLGeometry` class.