Initial authentication/authorization module

[rgwozdz]

Create an authentication module that:

• decorates a provider model prototype with three functions: authenticationSpecification, authenticate, and validateToken.
• allows users to pass in there own credential-checking method

[rgwozdz]

One higher-level question in response to the "registration" suggestion:

const auth = require('koop-auth-direct')({options})
koop.register(auth)
=> registered auth plugin for all providers

Do we want to have auth-plugin automatically applied to all registered providers? Or do we want a more explicit registration that looks for a property on the Provider? (e.g. auth: true).

[rgwozdz]

Another higher level question. Do we want to allow different identity-management approaches across providers? For example, say you have a koop instance with two providers, and you want to have different sets of users authorized for each provider. If we allow auth to be implemented differently across providers (e.g. different validateCredentials function per provider) we could support this use-case.

[rgwozdz]

@dmfenton - refactor as per discussion earlier today. 8718c2f