Implement standard DOM events

[georgecrawford]

As well as supporting the existing JS event listener callback (scroller.addEventListener()), it'd be great if FTScroller fired standard DOM events on the scroller DOM node as well. This would enable external modules which don't have access to the scroller instance in memory - for example test suites - to listen to (and indeed, synthesise) FTScroller events on an existing node. They might need to be namespaced, as the scroll event could conflict with the native browser scroll.