Uncomment ES6 module syntax

This removes the special comment part of the ES6 module syntax,
opting to enable ES6 module syntax by default.

It also appends `.js` to all import paths to better support in-browser
loading.

core/base64.js

@@ -112,4 +112,4 @@ var Base64 = {
     }
 }; /* End of Base64 namespace */
 
-/* [module] export default Base64; */
+export default Base64;