use Yii formatted URL

also slight changes in `widgets/JsTreeWidget.php`:
- rename `Yii_js.csrf` to `JsTreeBehavior.csrfToken`
- remove `Yii_js` (its empty now)
- use `json_encode()` to format the entire `JsTreeBehavior` array
- only call `Yii::app()->clientScript` once, then assign to variable