Support override of JsTree options from widget

For more flexibility I thought it would be nice to be able to set any JsTree option directly when calling the widget in a view, so i've added an $options attribute to the `JsTreeWidget` class. 
Any options set there will be merged to the default options defined in `js_plugins/jstree.behavior.js` using JQuery's `$.extend(true, {}, {});`, meaning if you define JsTreeWidget::$options, the defaults will not be lost, but can only be overriden or extended. 

Example: 

```php
<?php
$this->widget('application.widgets.JsTreeWidget', array(
	#...
	'options' => array(
		'html_data'=>array(
			'ajax' => array(
				'url' => $this->createUrl('fetchTree', array('id' => $_GET['id'])),
			),
		),
		'types' => array(
			'max_children' => 1,
		),
	),
	#...
));
?>