lib: issuing deprecation warning for freelist

As per the dicussion in nodejs#569,
this patch issues a deprecation warning when freelist modeule is
required.

Author: thefourtheye

lib/freelist.js

@@ -1,3 +1,6 @@
 'use strict';
 
+const util = require('internal/util');
+
 module.exports = require('internal/freelist');
+util.printDeprecationMessage('freelist module is deprecated.');