Add TimedJSONWebSignatureSerializer

Hi,

this adds a TimedJSONWebSignatureSerializer that makes use of 'exp' as specified in http://self-issued.info/docs/draft-ietf-oauth-json-web-token.html#expDef to encode the expiry time. This makes the expire time self contained so there's no need to pass in a `max_age` or `expires_in` when deserializing.