Create one or many workers.

def add(self, nb = 1, name = None):
        """
        Create one or many workers.
        """
        for x in xrange(nb):
            self.count_lock.acquire()
            self.shared['workers'] += 1
            xid = self.shared['workers']
            self.kill_event.clear()
            self.count_lock.release()
            w   = WorkerProc(xid, self.queue, self.shared, self.tasks, self.life_time, self.count_lock, self.kill_event)
            w.name = self.get_name(xid, name)
            w.start()