Add an element for tracking zoom animations. Use it for trying to stop a map zoom. Still doesn't quite work right. Stopping a zoomPan or PanAnimation does work however. Code is all cludgy :)

refs #2560
Stop animating before starting a ZoomPan, otherwise they both run at the same time.
stopAnimation -> stop
Getting closer. project and unproject based on the current zoom seems to work in most cases.
listen to moveend instead so we know where we are after a map drag
Move animated zoom stopping in to the place it belongs
Move the proxy in to Map.ZoomAnimation as it is the only place that uses it. Use it to detect zoom animation finishing.
Push getTransform up to L.DomUtil.getTransform rather than having it in ZoomAnimation and PosAnimation
Removing comment out code and not needed todos
Merge pull request #2563 from danzel/easey

Add stop animation support