use inherits module

Instead of inline code, use inherits module. Works better with browsers
and provides es < 5 support.

Author: defunctzombie

package.json

@@ -18,6 +18,9 @@
   "scripts": {
     "test": "node test/node/*.js"
   },
+  "dependencies": {
+    "inherits": "2.0.1"
+  },
   "devDependencies": {
     "zuul": "~1.0.3"
   },

util.js

@@ -560,17 +560,7 @@ exports.log = function() {
  *     prototype.
  * @param {function} superCtor Constructor function to inherit prototype from.
  */
-exports.inherits = function(ctor, superCtor) {
-  ctor.super_ = superCtor;
-  ctor.prototype = Object.create(superCtor.prototype, {
-    constructor: {
-      value: ctor,
-      enumerable: false,
-      writable: true,
-      configurable: true
-    }
-  });
-};
+exports.inherits = require('inherits');
 
 exports._extend = function(origin, add) {
   // Don't do anything if add isn't an object