Added the underscore node_module.

node_modules/underscore/cjs/indexBy.js

@@ -0,0 +1,9 @@
+var _group = require('./_group.js');
+
+// Indexes the object's values by a criterion, similar to `_.groupBy`, but for
+// when you know that your index values will be unique.
+var indexBy = _group(function(result, value, key) {
+  result[key] = value;
+});
+
+module.exports = indexBy;