Submitting the "weienwang" branch/solution to ChangingBackground

Neat challenge! The main two hangups I had were the following:

- Trying to animate the backgroundColor property of the underlying UIWindow, which wasn't cooperating. This was resolved by animating the alpha properties of two UIImageViews

- An odd stutter effect in the ViewController push from First to Second, which seems to be an issue that some are having with [UIColor clearColor] backgroundColor UIViews. This was resolved by adding the "Retro" category on UINavigationController to get an olden-days-style animation.