Hangup method now prevents future value additions.
Hangup method now prevents future value additions.

 * A new state was created for tracking whether or not the hangup method was
   called.
 * Hangup now prevents further values from being added to the blackboard,
   incase it was done in the middle of the loop on ->put...and just, in
   general.

ssmccoy/Async-Blackboard,ssmccoy/AnyEvent-Blackboard