Completion blocks for navigation controller animated operations

@ndonald2 @arrouse Just a little convenience category on UINavigation controller
This is probably a race condition. If this is not animated, the delegate will be notified immediately of the pop, before this method even returns. So the helper's block will have already been called and setting the array of popped view controllers will not matter.

Better to just get the last object in the current view controller array and set it before calling pop.