Added block callback for prev/next changes

I added a block callback that fires whenever the Next/Previous buttons are pressed, in order to get XCDFormInputAccessoryView to play nice with [TPKeyboardAvoiding](https://github.com/michaeltyson/TPKeyboardAvoiding).

I found that I could press Next until the active text field was way offscreen, and needed to fire TPKeyboardAvoiding's `-scrollToActiveTextField` method to move it into the visible area. 

If there's already an easy way to do this I totally missed it...but this callback might be useful for other things as well.