GKImageCropViewController: Move the logic to hide the NavigationBar  from viewDidLoad to viewWillAppear:animated:

I found that when pushing a GKImageCropViewController onto a UINavigationController stack that is not a UIImagePickerController the animation is off. Presumably UIImagePickerController is doing something in it's viewWillDissapear:animated: implementation.

By moving the line to hide the NavigationBar to viewWillAppear:animated: GKImageCropViewController will animated well when we push it onto a UIImagePickerController or a regular UINaviationController stack.