fix #179 :
add filter(predicate, rejectedStream)
add Reactor.ReplyToEvent.copy to produce a proper ReplyToEvent copy
add CallbackEvent to attach consumer callback on an event
add Deferred.accept(value, callback) to call the downstream Composable with a CallbackEvent
add Stream.callback() operation to define callback point, testing if the event is of type CallbackEvent and triggering its callback
modify Stream,Deferred and Composable to correctly handle events, rehydrating event headers and replyTo if necessary