Create one or many workers.

def add(self, nb = 1, name = None, xid = None):
        """
        Create one or many workers.
        """
        for x in xrange(nb):
            self.count_lock.acquire()
            if self.workers >= self.max_workers:
                self.count_lock.release()
                continue
            self.workers += 1
            if xid is None:
                xid = self.workers
            self.count_lock.release()
            self.kill_event.clear()
            w = WorkerThread(xid, self)
            w.setName(self.get_name(xid, name))
            w.start()