title: UIAlertView showing under KeyWindow's subview
body: <p>I've add a custom view on keyWindow. How can I show an UIAlertController upon the custom view? (Right now the alert just pop under my custom view.)</p>
  
  <pre><code>let customView = CustomView()
  UIApplication.sharedApplication().keyWindow!.addSubview(customView)
  
  let alert = UIAlertController(title: NSLocalizedString("Error", comment: ""), message: "", preferredStyle: .Alert)
  alert.addAction(UIAlertAction(title: "OK", style: .Default, handler: nil))
  presentViewController(alert, animated: true, completion: nil)
  </code></pre>