Add zoom events

Related to #114.

Sample usage for the VolumeViewer:

```javascript
BrainBrowser.events.addEventListener("zoom", function(volume, panel) {
    console.log(panel.zoom);
});
```

Sample usage for the SurfaceViewer:

```javascript
BrainBrowser.events.addEventListener("zoom", function(viewer, zoom) {
    console.log(zoom);
});
```

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I do however have an issue: the `zoom` level for the SurfaceViewer is always either 0.95 or 1.05. I never get any other value. The camera really zooms in or out, but the zoom level never changes…
Remove `viewer` object as argument, since we don't pass it to any other handlers (we might rethink this later).