Added the underscore node_module.

node_modules/underscore/modules/groupBy.js

@@ -0,0 +1,8 @@
+import group from './_group.js';
+import has from './_has.js';
+
+// Groups the object's values by a criterion. Pass either a string attribute
+// to group by, or a function that returns the criterion.
+export default group(function(result, value, key) {
+  if (has(result, key)) result[key].push(value); else result[key] = [value];
+});