Add volumeloaded event

related to #114

Usage example, in `volume-viewer-demo.js`:
```javascript
BrainBrowser.events.addEventListener("volumeloaded", function(volume) {
  console.log("volumeloaded", volume);
});
```
Result:
![image](https://cloud.githubusercontent.com/assets/1788596/4076387/b59dbe2a-2eb9-11e4-992a-719a74b39940.png)