Allow configuration of JWT for access token on a per client basis

[nynymike]

It would be nice if oxAuth could be configured to return a signed JWT access token from the /token endpoint that has the following claims (per https://tools.ietf.org/html/rfc7662#section-2.2)

1. aud
2. sub
3. iat
4. scope
5. iss
6. exp

We also need another client configuration parameter for access_token_signing_alg

Note this token should not be prefixed (see #804)

[yuriyz]

Client have:

• access_token_as_jwt - boolean value to indicate whether return access_token as jwt or not (default value is false);
• access_token_signing_alg - signing algorithm used to sign jwt.

[yuriyz]

JWT access_token are returned as bearer thus introspection token_type=bearer.
Authorization: Bearer eyJhbGci...<snip>...yu5CSpyHI. It will make client code consume jwt tokens as bearer and at the same time RP that cares about JWT can just parse it.

[yuriyz]

done in 3.1.4 and master