Add Type Definitions, bump to version 0.2.1

[FrostKiwi]

Adds Type Definitions, Solves #3 and complaints from typescript like:

Could not find a declaration file for module 'uint8-to-base64'. './node_modules/uint8-to-base64/cjs/index.js' implicitly has an 'any' type.
  Try `npm i --save-dev @types/uint8-to-base64` if it exists or add a new declaration (.d.ts) file containing `declare module 'uint8-to-base64';`ts(7016)

Surprised to find the browser's Uint8Array.prototype.toBase64() not widely supported and don't want to have a function in my code solving something I expect to be a standard, so I found this package and found it really useful. Typescript errors annoyed me, so I made this PR.