Added predefined slugs to avoid

Usage:

public function behaviors(){
return array(
'sluggable' => array(
'class'=>'ext.behaviors.SluggableBehavior.SluggableBehavior',
'columns' => array('title'),
'predefined' => array('search, user'),
'unique' => true,
'update' => true,
),
);
}

Predefined: a list of slugs will be avoided. In this example, a title
"Search" will never generate the slug "search". The first one will be
named "search-1"