Class Deferred

Controller --+
             |
  Deferred --+
             |
            Deferred

wraps up a Stage with a Deferred interface

In this version, the results of the Stage are used to 
construct a list of results and then sent to deferred.  Further,
in this version Cooperate is implemented via reactor's callLater.

    from twisted.internet import reactor
    from twisted.flow import flow
    
    def res(x): print x
    d = flow.Deferred([1,2,3])
    d.addCallback(res)
    reactor.iterate()