Visibility property for TbButton

I just thought it might be quite a tidy thing to have a visible property on the TbButton class, so you could easily hide and unhide buttons based on roles etc for a particular button.

you could thus have something like:
```php
<?php $this->widget('bootstrap.widgets.TbButton',array(
				'label' => 'Widget Report',
				'type' => 'success',
				'size' => 'medium',
				'url'  =>  $this->createUrl('affiliate/widgetreport',array('id'=>$data->id)),
				'visible'=>Yii::app()->user->checkAccess('manageWidgets'),
));?>
```