Test window with device orientation.

This is following the discussion in https://github.com/facebook/ios-snapshot-test-case/issues/29. I am not sure whether/what should be merged from here, however it might be useful for posterity.

I've modified the Demo app to display a red square on a yellow background and to support rotation of the device (rectangles of different size shown). I've added tests for that, which capture the window and not the view. This seems like an effective strategy in testing that view controllers render something meaningful.