Validate against draft 20

[progrium]

http://tools.ietf.org/html/draft-ietf-oauth-json-web-token-20

Not sure if contributions have kept us up to date, but we should double check and if it is update to date, the readme could use an update as to which spec it follows.

[Foxandxss]

It needs to add the claims, (there are 7 registered ones). At least exp should be implemented when possible. That makes the token expire within a time so the next time they are used, an exception is thrown :)

[excpt]

We have now all claims implemented and all algorithms. I think we can close this issue now. :)