title: jQuery draggable revert : function() can't return "invalid"
body: <p>So I want a piece of code executed when a draggable is reverted because it wasn't dropped on a droppable. Unfortunately return "invalid"; doens't seem to work, revert is acting like revert:true;
  Any suggestions how I can solve this?</p>
  
  <pre><code>revert: function () {
              //do some extra stuff...
              return 'invalid';
              }
  </code></pre>