basic JWK support

will work on JOSE_JWK::decode later.
JOSE_JWK::decode
more JWK tests
add Gemfile
support JOSE_JWK and Crypt_RSA as JWS sign/verify input
README update & spec tweaks
Merge pull request #1 from nov/feature/jwk-support

basic JWK support
update README
Merge pull request #18 from nov/master

JWK implementation