Call viewWillDisappear and viewWillAppear when scrolling to and away from view controllers

There was no way for a UIViewController to handle events where their views were no longer being shown by the XHTwitterPaggingViewer.

So I added a call to viewWill/Did(Appear/Disappear) on the corresponding view controllers when the user swipes either way on the XHTwitterPaggingViewer.

Now you can do things like have a UITextField become first responder when the user swipes to a view.