Remove base64 submodule and replace with new NSData methods

As noted in: https://github.com/nicklockwood/Base64 

"In the iOS 7 and Mac OS 10.9 SDKs, Apple introduced new base64 methods on NSData that make it unnecessary to use a 3rd party base 64 decoding library. What's more, they exposed access to private base64 methods that are retrospectively available back as far as IOS 4 and Mac OS 6."