docs: function mapSeries(arr, iterator) {
  		var self = this,
  			length = arr.length,
  			index = 0;
  
  		return Promise.reduce(arr, function(values, item) {
  		    return Promise.resolve(iterator.call(self, item, index, length))
  			.then(function(value) {
  		        values[index] = value;
  				index++;
  		        return values;
  		    });
  		}, []);
  	}