Add 'iat/ aud/ sub/ jti' support for ruby-jwt

[ZhangHanDong]

for iat check support:

example_payload = {"hello" => "world", "iat" => 1425917209}
example_secret = 'secret'
example_jwt = 'eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJoZWxsbyI6IndvcmxkIiwiaWF0IjoxNDI1OTE3MjA5fQ.m4F-Ugo7aLnLunBBO3BeDidyWMx8T9eoJz6FW2rgQhU'
decoded_payload = JWT.decode(example_jwt, example_secret, true, {iat: true})

or

begin
   example_payload = {"hello" => "world", "iat" => 1425917209}
rescue JWT::InvalidIatError
   # do something
end

aud check support

example_payload = {"hello" => "world", "aud" => "url:pnd"}
example_payload2 = {"hello" => "world", "aud" => ["url:pnd", "aud:yes"]}
example_secret = 'secret'

example_jwt = 'eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJoZWxsbyI6IndvcmxkIiwiYXVkIjoidXJsOnBuZCJ9._gT5veUtNiZD7wLEC6Gd0-nkQV3cl1z8G0zXq8qcd-8'
example_jwt2 = "eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJoZWxsbyI6IndvcmxkIiwiYXVkIjpbInVybDpwbmQiLCJhdWQ6eWVzIl19.qNPNcT4X9B5uI91rIwbW2bIPTsp8wbRYW3jkZkrmqbQ"

decoded_payload = JWT.decode(example_jwt, example_secret, true, {aud: "url:pnd"})
decoded_payload2 = JWT.decode(example_jwt2, example_secret, true, {aud: "url:pnd"})

for sub support

example_payload = {"hello" => "world", "sub" => 'subject'}
example_secret = 'secret'
example_jwt = 'eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJoZWxsbyI6IndvcmxkIiwic3ViIjoic3ViamVjdCJ9.QUnNVZm4SPB4vP2zY9m1LoUSOx-5oGXBhj7R89D_UtA'
decoded_payload = JWT.decode(example_jwt, example_secret, true, {sub: 'subject'})

for jti support

In order to ensure jti uniq for every jwt:

1. Use the current timestamp (iat) as the nonce.
2. Use the md5 hash of the jwt.

example_payload = {"hello" => "world", "iat" => 1425917209, "jti" => Digest::MD5.hexdigest("secret:1425917209")}
example_secret = 'secret'
example_jwt = 'eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJoZWxsbyI6IndvcmxkIiwiaWF0IjoxNDI1OTE3MjA5LCJqdGkiOiI1NWM3NzZlMjFmN2NiZDg3OWMwNmZhYzAxOGRhYzQwMiJ9.ET0hb-VTUOL3M22oG13ofzvGPLMAncbF8rdNDIqo8tg'

 decoded_payload = JWT.decode(example_jwt, example_secret, true, {jti: Digest::MD5.hexdigest("secret:1425917209")})

[excpt]

Awesome!

Thanks a lot! 🎉

[ZhangHanDong]

no problem， i love json web token and i 'll be used it in my project.