Added two additional delegate methods that are called when the Next and Previous buttons are tapped in the input accessory view.

I added the delegate methods form:shouldMoveToNextField:fromCurrentField: and form:shouldMoveToPreviousField:fromCurrentField:

They are called when the Next and Previous buttons are tapped in the input accessory view.

The methods can be handy if you have a need to react in some way to the buttons being tapped, and they also allow you to override the default behavior of selecting the next first responder if you have a reason to do so.