Status notification

[David Hull <dmh@tibco.com>]

(Is there already an issue open for this?)

Whether or not we try to use WSN for its own administration, there needs 
to be some way for the system to notify the outside world of 
subscription lifecycle events.  This could be as simple as an optional 
wsn:StatusReference element in the Subscribe message.  This endpoint 
would receive notification of pause/resume/modify/destroy events.  It 
might even be a place to send SubscribeResponse/fault.