added collapse

Example of usage:

```php
echo Collapse::widget(array(
	'items' => array(
		'Collapsible Group Item #1' => array(
			'content' =>  'Anim pariatur cliche...',
			'contentOptions' => array('class' => 'in')
		),
		'Collapsible Group Item #2' => array(
			'content' =>  'Anim pariatur cliche...',
		)
	),
));
```
closes #332