Add support for UIPickerViews in Subliminal

UIPickerViews are under a different Window other than keyWindow, so I had to find the other one.  Otherwise it is pretty straightforward.  I'm leaning on the UIA methods a bit more heavily for interacting with the element, but open to suggestions.

Added one fairly comprehensive test case.
`NSJSONSerialization` will throw an exception if invoked with `nil` data. Let's make our potential failure ;D a little more explicit:

```objc
NSData *jsonDataFromString = [responseString dataUsingEncoding:NSUTF8StringEncoding];
NSAssert(jsonDataFromString, @"`%s JavaScript is malformed.", __PRETTY_FUNCTION__);
```