Don't pollute global scope with atob and btoa

[moll]

Hey,

I realize it's meant to be a polyfill, but I don't think polluting the global scope with atob and btoa as a side-effect of requiring the module is a good idea. If they ever come to V8 as a runtime, then great, but till then people using them will have to import them from somewhere manually anyways and a side-effecty require statement just ain't cool. :)

Otherwise, thanks for the module!

[LiraNuna]

They seem to be unused by the project?

node-webcrypto-ossl$ git grep btoa
lib/webcrypto.ts:// Fix btoa and atob for NodeJS
lib/webcrypto.ts:g.btoa = (data: string) => new Buffer(data, "binary").toString("base64");
node-webcrypto-ossl$ git grep atob
lib/webcrypto.ts:// Fix btoa and atob for NodeJS
lib/webcrypto.ts:g.atob = (data: string) => new Buffer(data, "base64").toString("binary");

They are only defined on https://github.com/PeculiarVentures/node-webcrypto-ossl/blob/master/lib/webcrypto.ts#L9-L12