Added dragDidSucceed, dragDidCancell, source.dragSlideBackDidEnd callbac...

Per discussion with @dcporter, adding new drag callbacks:
* dragDidSucceed - called right before dragDidEnd, if the drag successfully finishes
* dragDidCancel - called right before dragDidEnd, if the drag was cancelled
* dragSlideBackDidEnd - called at the end of the slide back animation, if slide back is enabled. Note that this will be called way after dragDidEnd