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.
What if we made `SLDatePicker` re-implement `SLPickerView`'s interface without subclassing? Since it matches a different type of object. It could forward the interface to an internal `SLPickerView`. Here's a patch showing how that could work: http://cl.ly/code/1F2V0i3J1L1n.