title: Setting the class of a dynamically created paper-button doesn't work
body: <p>I have the following code that creates a paper-button dynamically.  It works well but I can't figure out how to set the class.  </p>
  
  <pre><code>var button = document.createElement('paper-button');
  button.raised = true;
  Polymer.dom(button).textContent = 'dynamic';
  Polymer.dom(button).innerHTML = "New Button Text";    
  $(".notificationCollection").append(button);
  </code></pre>
  
  <p>This does not work:</p>
  
  <pre><code>Polymer.dom(button).class = "notificationBarButton";
  </code></pre>
  
  <p>I couldn't find anything in the documentation.  Thanks. </p>