Include Client ID (kid) parameter in the JWT header

[BVMiko]

Currently, Oauth2\Encryption\FirebaseJwt::encode has a different signature from OAuth2\Encryption\Jwt::encode; the FirebaseJwt function has an additional $keyId parameter. This parameter, when included, is passed into the kid parameter of the JWT header.

Unfortunately, neither of the two classes which use it (OAuth2\OpenID\ResponseType\IdToken and OAuth2\ResponseType\JwtAccessToken) include the parameter by default, so a lot of code is required to get this parameter added into the JWT header.

This PR modifies the signature of OAuth2\Encryption\EncryptionInterface::encode to include the $keyId parameter, thus standardizing between the two provided encryption implementations to offer a way to incorporate the kid parameter. It also modifies the two ResponseTypes which use the encode method to pass the $client_id.

There is also one more related issue included in this PR: modern versions of firebase/php-jwt use namespaces, while the references in Oauth2\Encryption\FirebaseJwt do not. I've modified this class to use the proper namespace parameters, and also upgraded the minimum suggestion of firebase/php-jwt package from ~2.2 (released 2015-06-22) to ~3.0 (released 2015-07-22).

[BVMiko]

I also want to reference to issue #363; though this PR doesn't offer a way to modify the JWT header with arbitrary data; it does automatically provide the kid parameter; which I think may solve the issue for most.

[menturion]

+1