Add Base64.js 0.1.3

Base64.js is a polyfill for browsers which don't provide window.btoa and window.atob.

Repository: https://github.com/davidchambers/Base64.js
Files downloaded from https://raw.github.com/davidchambers/Base64.js/master/base64.js and https://raw.github.com/davidchambers/Base64.js/master/base64.min.js.