title: Vaadin 7 Refresher addon doesn't work
body: <p>I'm migrating from Vaadin 6 to Vaadin 7. <br>
  After changing Refresher for Vaadin 6 to Refresher for Vaadin 7, It seems that I should just change addComponent(refresher) to addExtension(refresher). But unfortunatelly when I did it I recieved an empty page instead of my component. Can anybody help? I don't fully understand what I should do..</p>