Fix JQuery Sortable to Operate with Meteor UI (Blaze)

As you can see in the DIFF view, my fix is largely limited to functions "_clear" and "cancel" (plus an additional property in the domPosition object). The idea is to take special care so that when a JQuery Sortable drag/drop operation is cancelled, the node that was being dragged is returned to precisely the same position in the DOM relative to its original siblings. In my own system, this seems to work beautifully. Since the DOM is unchanged, Meteor UI is able to properly perform "differencing" in the DOM to update it correctly in response to state change notifications from Mongo.