Adding kid to the header of the JWT token issued by light-java.

[stevehu]

This is necessary to support multiple public key certificates on light-oauth2. Also, for the resource service to verify a in coming JWT token, it won't need to iterate every key available but just pick the right key to verify the token. If token verification module cannot find the right key locally, it will call the light-oauth2 key service to get the public key certificate by kid.