Local cache for getClosestElementWithVirtualBinding

While working on #4458 i noticed, that most of the cpu is spent by $.data in order to handle the v-events. This is a simple patch to work with a local-cache for 'getClosestElementWithVirtualBinding'.

This improves the event-handling for events which are triggered repetitively from the same source, like e.g. when dragging a slider-handle.

I've added no special unit-test for this, but all existing ones are still passing in Chrome 19 & ipad2/ios 5.1.1.