Fix atob to match browser implementations

Test case:
atob('/A==') === String.fromCharCode(252) should be true

index.js

@@ -2,7 +2,7 @@
   "use strict";
 
   function atob(str) {
-    return new Buffer(str, 'base64').toString('utf8');
+    return new Buffer(str, 'base64').toString('binary');
   }
 
   module.exports = atob;