透明的UIViewController

iOS 7 之前，只需要这样一句话：

self.window.rootViewController.modalPresentationStyle = UIModalPresentationCurrentContext;

iOS 8之后，还需要这样设置

if ([UIDevice currentDevice].systemVersion.floatValue == 8.0) {
      self.modalPresentationStyle = UIModalPresentationOverCurrentContext;
}