Add options for claim-specific leeway

[EmilioCristalli]

This PR intends to fix #129 by adding one independent leeway option for each claim.

I tried to make it backwards compatible by keeping the "global" leeway option (which is overwritten by each claim-specific leeway if they are present).

Let me know what you think

[excpt]

Hey @EmilioCristalli

Thanks for the PR.

Can you drop the leeway for the iat claim, please?

https://tools.ietf.org/html/rfc7519#section-4.1.6

The RFC does not allow leeways for that claim.

Only the exp and nbf claims are allowed to have a leeway.

[EmilioCristalli]

Hey @excpt , I was also thinking about that, and my two concerns are:

1. Backwards compatibility: if we remove the leeway for iat we change a functionality people can be relying on, is it worth doing it for 2.0?
2. iat validation: I might be misinterpreting the RFC, but it seems like it doesn't talk about validating iat, it's like its purpose is to determine the age of the token, so maybe that's why it doesn't talk about a leeway for this claim.

In my opinion, the iat validation is an extra feature provided by the gem, so I would either remove completely this validation, or keep it with a leeway, as the rest of time sensitive validations have a leeway.

[excpt]

After discussing the changes in the PR thread. I accept these changes. Thank you very much. :)