Overwrite UIWindows's standard rootViewController getter for iOS 7 status bar handling

As of iOS 7 an application is looking for the top most window (regarding to it's level) to access it's rootViewController to access methods like 'preferStatusBarHidden'. Because MTStatusBarOverlay must has a higher windowLevel than the keyWindow (if the keyWindow is fullscreen) the application takes MTStatusBarOverlay rootViewController which makes the keyWindow's view controllers unable to modifiy the visiblity and style of the status bar under iOS 7.

Therefore I've overwritten the MTStatusBarOverlay's rootViewController getter to return the keyWindow's rootViewController.