Bug 1113117 - Show "someone joins to a Room" notifications when the user is in background mode

See bug https://bugzilla.mozilla.org/show_bug.cgi?id=1113117 please. Action happens at https://github.com/jaoo/firefoxos-loop-client/tree/1113117.
or better
```js
notification.onclose = {
  document.removeEventListener('visibilitychange',  onVisibilityChange);
  notification.onclose = notification.onclick = null;
};