In browser loader, refer to global object as 'this', not 'self'

Closes #41

util/walk.js

@@ -3,7 +3,7 @@
 (function(mod) {
   if (typeof exports == "object" && typeof module == "object") return mod(exports); // CommonJS
   if (typeof define == "function" && define.amd) return define(["exports"], mod); // AMD
-  mod(self.acorn.walk = {}); // Plain browser env
+  mod((this.acorn || (this.acorn = {})).walk = {}); // Plain browser env
 })(function(exports) {
   "use strict";