title: Polymer behavior listen to child property change
body: <p>I want a build a data-type behavior that listen's to a property update.
  What's the best way to listen to child property changes?</p>
  
  <pre><code>function do_something_when_property_updates(property_name) {
    return {
      // How to listen to property?
      properties: {
         property_name: {
           // This behavior doesn't (and shouldn't) own the property
           observer: foo
         }
      },
      foo: function(value) {
        // Update handler
      }
    };
  }
  
  Polymer({
    is:'test-element', 
    properties: {
      bar: {
        type: Object
      }
    },
    behaviors: [do_something_when_property_updates('bar')]
  });
  </code></pre>