Auth refactor

[philippeitis]

Since we're moving on to the next API version, I'm revisiting this change as well.

PR includes:

• Documentation on how to use authentication, and how to implement custom authenticators as needed
• More flexible authentication error types
• Optional yup-oauth2 (included by default)
• Optional token for APIs which optionally require token.

I'd also like to see about entirely removing the yup-oauth2 dependency for (and auth argument in Hub constructor) for unauthenticated APIs