Make DropDown support divider class.

```php
Dropdown::widget(array(
	'items' => array(
		array('label' => 'li1', 'url' => $this->context->createUrl('site/index')),
		array('label' => '', 'options' => array('class' => 'divider')),
		array('label' => 'li3', 'url' => $this->context->createUrl('site/index')),
	)
)
```