add hook::alterCalculatedMembershipStatus

Per http://forum.civicrm.org/index.php/topic,32013.msg136959.html & IRC add hook to allow alteration of membership status. This includes passing membership type into this function so it can be passed to the hook (which is also a step towards Andrew's ideas as to how membership status by type could be added to core. The backend functionality of Andrew's idea makes sense but the UI & rules on saving membership statuses might need some work)