Decode url-safe style base64

In trying to help improve [buffer](https://github.com/feross/buffer/issues/44)'s compatibility with node I came across this:

https://github.com/joyent/node/commit/b9127eb0a5d64a2317197e3b4956bdd7450523da

It might be nice to have this here, but it could probably also be done in buffer, open to suggestions.