Adding Behavior for Slider.js.

Sorry if I wasn't clear about this, but you'll need to fire this event yourself, as Slider doesn't do it for you. In your `initialize` function, after you call `this.parent`, you'll need to add this line:

```javascript
this.drag.addEvent('start', this.fireEvent.bind(this, ['start']));
```

And, for consistency, I'd name your event here onStart. This is because Slider doesn't provide an event for this (as Drag does), but instead binds it's `draggedKnob` method to both the start and drag events:

https://github.com/mootools/mootools-more/blob/master/Source/Drag/Slider.js#L85