Work around incorrect element detection on iOS 6 in transitioning/scrolling layers (fixes #57)

iOS 6.0 (and potentially beyond) has a bug where event.target gives an incorrect element if that element is within a parent that is currently being scrolled or CSS transitioned.  This works around the problem by performing manual detection in these circumstanced by using document.elementFromPoint with coordinates derived from the event, which works if preceded by an event.target.