Alter CI's default session to Work with AJAX!

this class alters CodeIginiter's default session class by the following logic:
to make codeigniter able to solve problem of Session rotation on
ajax requests. by default code igniter updaes session_id every five minutes
when the ajax requestS are made on the time of update cause session loss
 and user is logged out automatically, to solve this problem we extend CI's 
 default sess_update() method to do not update session_id if is_ajax_request()
 @author bigsinoos