public function start($taskName, $repeat = null)
    {
        $task = new Task();
        $task->name($taskName);
        if ($repeat) {
            $task->repeat($repeat);
        }
        if (isset($this->_tasks[$taskName])) {
            throw new Exception("Task {$taskName} is already defined.");
        }
        $this->_tasks[$taskName] = $task;
        $task->start();
        return $task;
    }