title: <p:commandButton> with icon image not showing
body: <p>I have been struggling this for a while. I am using primefaces 3.5. I have a command button with icon attribute. But the image seems not showing. I am not sure if I put the image url correctly. I tried a few path. But still not working. In firebug I can see the icon.</p>
  
  <p>My image residing on /BelsizeWeb/WebContent/theme/primefaces-aristo/images
  CSS residing on /BelsizeWeb/WebContent/theme/primefaces-aristo</p>
  
  <p><strong>theme.css</strong>       </p>
  
  <pre><code>.ui-icon-excel {
      background-image: url("images/toexcel.gif");
  }
  </code></pre>
  
  <p><strong>xhtml</strong></p>
  
  <pre><code>&lt;link type="text/css" rel="stylesheet"
    href="#{request.contextPath}/theme/primefaces-aristo/theme.css" /&gt;
  
  &lt;p:commandButton type="submit"
                   styleClass="commandExButton_image" id="cr1001_command_toexcel"
                   icon="ui-icon-excel"
                   action="#{pc_Cr1001.doCr1001_command_toexcelAction}" ajax="false"&gt;
                &lt;/p:commandButton&gt;
  </code></pre>