code: function(map) {
        var arr = map ? this.map(map) : this;
        return arr.length > 0 ? arr.sum() / arr.length : 0;
      }
docs: *
  @method average([map])
  @returns Number
  @short Averages all values in the array.
  @extra [map] may be a function mapping the value to be averaged or a string acting as a shortcut.
  @example
  
  [1,2,3].average()                           -> 2
  +   [{age:35},{age:11},{age:11}].average(function(n) {
  return n.age;
  });                                         -> 19
  [{age:35},{age:11},{age:11}].average('age') -> 19
  
  *