Added token_translator and STACK-2 compat

Added a `token_translator` service. It accepts the OAuth token and returns a value that is to be treated as a STACK-2 token.

The `oauth.token` is left for BC and because it might make sense to be able to look specifically for it if you're not in an environment that cares about STACK-2. This can be changed if needed.

This is something that I'm using in stack-firewall stuff. It provides a way to translate the raw token value into something usable by the application if it is so desired.

I may still work on providing stack-firewall and/or stack-authentication integration in a bit. :)