複数タスクを配列でaddするメソッドです。
@memberof Bucks
@instance
@param {Array} tasks タスクの配列です。タスクの形式は addと同様です
@return {Bucks}
@example
var t1 = function t1(err, res, next) {
return 't1';
};
var t2 = funciton t2(err, res, next) {
return 't2';
};
new Bucks.waterfall([t1, t2]).end(function finish(err, ress) {
// ress => ['t1', 't2']
});

// same as
new Bucks.add(t1).add(t2).end(function finish(err, ress) {
// ress => ['t1', 't2']
});
function waterfall(tasks) {

            if (!isArray(tasks)) {
                throw new Error('tasks is not array.');
            }

            if (isArrayEmpty(tasks)) {
                return this.add(function passEmpty(err, res, next) {
                    next(null, []);
                });
            }

            // normalize and add each task
            for (var i = 0, l = tasks.length; i < l; i++) {
                tasks[i] = this._normalizeTask(tasks[i]);

                this.add(tasks[i]);
            }

            return this;
        }