title: Material UI <IconButton> not showing
body: <p>I am using <a href="http://www.material-ui.com/#/components/icon-button" rel="nofollow noreferrer">Material UI</a>, and I have the following: </p>
  
  <pre><code>import IconButton from 'material-ui/IconButton';
  &lt;IconButton iconClassName="muidocs-icon-custom-github" tooltip="bottom-right" tooltipPosition="bottom-right" /&gt;
  </code></pre>
  
  <p>When I am testing the tooltip, the icons are not showing, but the tooltip is showing on hover. Can someone help me create an example? Am I missing the icon URL?</p>