Improve scroll event generation to occur within the requestAnimationFrame where supported by the platform to aid draw coalescing when drawing in response to scroll events.  Also rearrange scroll event handling to make all positioning as a result of scrolling go through _scheduleRender for simplicity, and add a new getPositions() internal function to aid consistent output
Improve ftscroller scroll start & end performance by no longer adding a scrolling class by default, thus avoiding a relayout of scroller contents due to a container classname change.  This takes @kaij's https://github.com/ftlabs/ftscroller/pull/57 and tweaks it slightly to address Issue #57, except making the faster behaviour the new default.

To be able to do this some scroll handling has been tweaked, particularly around interrupting scrolls.  A new option (`scrollingClassName`) has been added for those who still want to change styles while scrolling is active.