Bug 847566 - Shim for mozSetMessageHandler and mozNotification. r=kgrandon

Yet another addition to the shims. This adds mozSetMessageHandlers and mozNotification. To trigger a message handler execute:

```javascript
navigator.mozTriggerMessageHandler('eventname', { payload: 'data' })
```

Note that handlers aren't auto-added. You have to start up the app once (f.e. SMS app), then you can go back to the homescreen, trigger the event and see the notification pop up that originates from the SMS application.

@vingtetun @kgrandon