Rename JWT claims for JWT Access Tokens: #338

Hey all,

Thank you for creating such a fantastic library. It has been very useful to me. This pull request is my attempt to finish addressing #338 for JWT Access Tokens. A couple of notes here:
- renamed `client_id` to `aud`
- renamed `expires` to `exp`
- renamed `user_id` to `sub`
- added the `iss` parameter from an optional `issuer` configuration value
- added the `iat` parameter for the time the JWT access token was issued

I also added a method that maps the decoded access token from the JWT claims format to the format expected by other parts of this library. Hope this is helpful!

- Jared