prevent default touch scroll action if page scroll is not intended

This fixes #94

Chrome mobile browser fires touchcancel 200ms after touchmove so sometimes dragging becomes jittery as the browser thinks the page is to be touch scrolled.

http://stackoverflow.com/questions/20412982/javascript-any-workarounds-for-getting-chrome-for-android-to-fire-off-touchmove

The fix checks to see if the px movement is outside a buffer and then calls preventDefault() on the event to enable touchmove to fire correctly.