add ondragstart / ondragstop event handlers

As observed in https://github.com/malihu/malihu-custom-scrollbar-plugin/issues/114 scrollInertia: 0 resulted in onScroll and onScrollStop not behaving as desired. This commit adds two new callbacks that behave in the way I was wanting: onDragStart and onDragStop. See http://www.terrafrost.com/autogroup/js/test2.html for a demo.