code: function(map, fn) {
        var arr = this, result = {}, key;
        arrayEach(arr, function(el, index) {
          key = transformArgument(el, map, arr, [el, index, arr]);
          if(!result[key]) result[key] = [];
          result[key].push(el);
        });
        if(fn) {
          iterateOverObject(result, fn);
        }
        return result;
      }
docs: *
  @method groupBy(<map>, [fn])
  @returns Object
  @short Groups the array by <map>.
  @extra Will return an object with keys equal to the grouped values. <map> may be a mapping function, or a string acting as a shortcut. Optionally calls [fn] for each group.
  @example
  
  ['fee','fi','fum'].groupBy('length') -> { 2: ['fi'], 3: ['fee','fum'] }
  +   [{age:35,name:'ken'},{age:15,name:'bob'}].groupBy(function(n) {
  return n.age;
  });                                  -> { 35: [{age:35,name:'ken'}], 15: [{age:15,name:'bob'}] }
  
  *