Clip to bounds to avoid allowing the child controllers to appear outside of the page view controller's bounds in situations where it does not fill the screen.
Added pageViewController:didActivateViewController: delegate method for notification at the moment the active controller changes.
Added transitioningDueToUserInteraction property to aid in determining whether the transition is due to user interaction or programmatic view controller changes. Particularly useful when the delegate wants to react only to changes made by user interaction rather than other factors.