Cancellable tween operations

[cmezak]

Added a BOOL flag to tween operation. when this is set to YES, the tween operation is cancelled.

Im using a tween to add animations when the user finished interacting with a collection view, but I need to cancel these animations if the user begins interacting with the view again before the animations finish.

[rivera-ernesto]

Needs update to be included in https://github.com/PRTween/PRTween.