code: 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;
          }
docs: 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']
  });