JWT API Update

[EternalDeiwos]

So far the signing and verification of tokens of all three JWT serializations is working as expected. I've also added a new example to demonstrate this.

Notable features:

• New API (the input-whatever-you-like-API)
• Function splitting for test-ability and general good design practices
• Standardised in-memory representation of all JWTs (and therefore standardised operations regardless of serialization.

This PR will be ready to merge after the following TODO list is complete:

• [x] Refactor static and member encode
• [x] Additional error checking
• [ ] Various todos scattered across the document to be investigated
• [x] JWD class refactor to new API
• [x] Converting between JWT and JWD serialisations (JWT to JWT is already done and working)
• [ ] Unit testing
• [ ] Documentation

The current plan for the JWD refactor is to have it extend from JWT as most of the functionality for JWD is already in JWT with very few overrides necessary. Please discuss here if this is going to be a problem for anyone.

I would really appreciate any active discussion and additional eyes on this. Please feel free to contact me via commenting here / email / slack with questions, suggestions and potential breakages.

[EternalDeiwos]

@christiansmith the tests have been updated to use the new API. No new tests have been added yet but I would say that this branch is ready to be merged in with smith-linklater-jws-signing.

There's still a lot of work to be done w.r.t. documentation, new unit tests and TODOs.

@dmitrizagidulin the API is now what I would call stable.

[dmitrizagidulin]

@EternalDeiwos brilliant, thanks! I'll give it a shot to update one of the oidc libs to use the new api.

[dmitrizagidulin]

Wait, so, since this PR is not into master, but into the smith-linklater-jws-signing branch.. can ya'll merge this linklater-jwt-api into smith-linklater-jws-signing and then make a PR into master with the end result?