Fix App#window to work with iOS7 style action sheets

`App.window` should return the application-level most likely used by the app. iOS7 has introduced implementation changes to how modal views work, which complicates the `UIWindow` hierarchy.

@matthewsinclair what do you think about this? basically `App.window` should return the expected window, thus `rootViewController`. Folks shouldn't have to do a bunch of checks every time, like #304. will merge when you take a look