def add(message)
      @services.each do |service|
        if service.can_send?(message)
          if @queued_messages[service].nil?
            @queued_messages[service] = []
          end
          @queued_messages[service] << message
          return true
        end
      end
      return false
    end
def add(message)
      @services.each do |service|
        if service.can_send?(message)
          if @queued_messages[service].nil?
            @queued_messages[service] = []
          end
          @queued_messages[service] << message
          return true
        end
      end
      return false
    end
Adds a PushMessage to the queue
 @param message [PushMessage] the message to be added to the queue
 @return [Boolean] true if message was added to the queue (that is: if any of the registered services can handle the message), otherwise false
 @see PushMessage