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.
This block is asynchronous so var "i" could have been increased here. My suggestion would be to use 

```js
room.participants.forEach(participant => {bla bla})
````

here https://github.com/mozilla-b2g/firefoxos-loop-client/pull/418/files#diff-022ef6e8db3e7db4347a1e72beb42c0dR273