Make Fingertips a UIWindow category instead of a subclass

Previously you had to change the class of your application window to `MBFingerTipWindow` to make Fingertips available. This was a problem in those few cases where you do not instantiate the window yourself. If the window was a plain UIWindow, you could still change its class at runtime using `object_setClass`, but in the case the instantiated window was already some custom `UIWindow` subclass, you were out of luck.

I fixed this issue by turning Fingertips into a `UIWindow` category, thus solving this issue. I realize this is a major interface update, and I of course wouldn't mind if you decided to let this pull request die in peace.

Thanks!