title: Polymer paper-icon-button doesn't show an image
body: <p>I want to make a paper-drawer-panel that has a menu button on the left side that opens the navigation drawer. So in the main header panel I added the button, just like in the Polymer documentation:</p>
  
  <pre><code>&lt;paper-header-panel main&gt;
    &lt;paper-toolbar&gt;
      &lt;paper-icon-button icon="menu" paper-drawer-toggle&gt;&lt;/paper-icon-button&gt;
        &lt;div&gt;Extracurricular Activities&lt;/div&gt;
      &lt;/paper-toolbar&gt;
      &lt;!-- main content --&gt;
    &lt;/paper-toolbar&gt;
  &lt;/paper-header-panel&gt;
  </code></pre>
  
  <p>The problem is that the menu button doesn't have an image, but I correctly included the paper-icon-button. When I click on the place where the button should be I can se a normal ripple effect and the drawer opens, so the button must be there.</p>
  
  <p><a href="http://i.stack.imgur.com/l9IQ9.png" rel="nofollow">Here is a screenshot of the header, the menu button should be at the left of the header text</a></p>
  
  <p>Does anybody have an idea what I'm doing wrong?</p>