btoa() and atob() #3462

All major browsers expose the btoa and atob globals for encoding ASCII strings as base64 and vice versa. It'd be beneficial for the "isomorphic" javascript topic if we'd provide these too:

• https://developer.mozilla.org/en-US/docs/Web/API/WindowBase64/btoa
• https://developer.mozilla.org/en-US/docs/Web/API/WindowBase64/atob

As for the implementation, we can't Buffer('string').toString('base64') because that would encode unicode while atob and btoa must per spec throw on charcodes > 255. https://github.com/mathiasbynens/base64 looks like a solid implemenation we could pretty much drop in.