Checkbox support for transforming it to a toggle button

Here is an examplre to transform a checkbox to a toggle button by the modification above:

```php
->add('isActive', 'checkbox', array(
	'widget_checkbox_label'=>'widget',
	'widget_form_group_attr'=>array(
		'class'		=> 'btn-group',
		'data-toggle'	=> 'buttons'
	),
	'label_attr'=>array(
		'class'		=> 'btn btn-primary'
	)
))
```