Add support for PS256, PS384 and PS512 algorithms (see http://self-issued.info/docs/draft-ietf-jose-json-web-algorithms-11.html#DefiningPSS)
Using salt len = hash len as per http://www.ietf.org/mail-archive/web/jose/current/msg02901.html
Tests added but I've not touched the README
Merge pull request #5 from davedoesdev/master

Add support for PS256, PS384 and PS512 algorithms