Accordion Morph

Added class for "Accordion" widget inspired by the [jQuery UI Accordion](http://jqueryui.com/accordion/).

### Example usage:

```javascript
var a = new lively.morphic.Accordion();
a.addSection('Foo').setFill(Color.red);
a.addSection('Bar').setFill(Color.green);
a.addSection('Baz').setFill(Color.blue);
a.openInWorld();
```

### Result:

![accordion](https://f.cloud.github.com/assets/479238/178683/9b2a9a7a-7bc5-11e2-889b-dc8d3510ef3d.png)