Make sure base64 encoding works on other platforms

[jakubkoci]

Module SignatureDecoratorUtils is using base64url package which apparently doesn't work on React Native. There is an issue about that in package repo brianloveswords/base64url#50.

I looked into the code and the problem is probably dependency on Buffer which is a globally available object in Node.js. I see two solutions:

1. Implement functionality on our own.
2. Use a different npm package with support for the web and React Native platform.

In both cases, I would suggest introducing a class JsonEncoder which would hide implementation details and provide the same API across the framework independently on a particular package.

[TimoGlastra]

This is fixed by #135