Add support for nbf and exp claims

[alexjab]

Hi, here is a PR for this issue (the nbf and exp claims).

Before merging, I have a question : what purpose do you intend for the noVerify option ? Should it skip signature validation and the new nbf and exp claims, or only the signature validation ?

If we choose the latter, I will need a little refactor of this PR...

For me, both of these options sound valid in their own way.

[hokaccha]

Before merging, I have a question : what purpose do you intend for the noVerify option ? Should it skip signature validation and the new nbf and exp claims, or only the signature validation ?

If noVerify is true, nbf and exp claims also should be skipped. Your code is correct 👍