Preserve object callback

As `dragend` unsets `dragging` we can't pass it back to the `drop` callback.

Preserve dragging and return it along with the droppable object (`this` is the first object returned by a callback -- see http://stackoverflow.com/questions/6016295/jquery-pass-parameters-when-using-call).

To implement use
`        $('.droppable').droppable({
          drop: function(dragged) {
            dragged.clone(false)
              .attr('draggable', false)
              .removeClass('draggable')
              .appendTo(this);
          }
        });
`