Add 'root' global variable as reference to sandbox

lib/module.js

@@ -400,6 +400,7 @@ Module.prototype._compile = function (content, filename) {
       sandbox.__filename  = filename;
       sandbox.__dirname   = dirname;
       sandbox.module      = self;
+      sandbox.root        = sandbox;
 
       Script.runInNewContext(content, sandbox, filename);
 
@@ -411,6 +412,7 @@ Module.prototype._compile = function (content, filename) {
       global.__filename = filename;
       global.__dirname  = dirname;
       global.module     = self;
+      global.root       = global;
       Script.runInThisContext(content, filename);
     }