Move photo selection from `clickedButtonAtIndex:` to `didDismissWithButtonIndex:` to fix iOS 8 iPad photo selection issue.
 > Warning: Attempt to present <UIImagePickerController>  on <ATSimpleImageViewController> which is already presenting (null)
http://stackoverflow.com/questions/24942282/uiimagepickercontroller-not-presenting-in-ios-8
Fixes IOS-661