[ARDrone][Video] Add 'deviceready' event and init() function

'deviceready' event is added to notify web page to play() the video.
Thus web app developer can show banner when no videos.
I prefer:
```javascript
if (msg.eventName == 'deviceready') {
    event = new ARDroneDeviceReadyEvent();
} else if (msg.eventName == 'newvideoready') {
  event = new ARDroneNewVideoEvent(msg.data);
}
```