message: support draggable on group
diff: @@ -20,7 +20,11 @@ Draggable.prototype = {
  _dragStart: function (e) {
  var draggingTarget = e.target;
  - if (draggingTarget && draggingTarget.draggable) {
  + // Find if there is draggable in the ancestor
  + while (draggingTarget && !draggingTarget.draggable) {
  + draggingTarget = draggingTarget.parent;
  + }
  + if (draggingTarget) {
  this._draggingTarget = draggingTarget;
  draggingTarget.dragging = true;
  this._x = e.offsetX;
  