Triggers after group is deleted.
~~~
<?php
function my_component_init(){
ossn_register_callback('group', 'delete', 'my_component_cb');
}
function my_component_cb($callback, $type, array $params){
//$params['entity'] contains OssnGroup instance
}
ossn_register_callback('ossn', 'init', 'my_component_init');
~~~
