Added claim for not before time (nbf) to coincide with issued at time.

[Eric2017a]

This ensures JWT will not be considered valid for times before it was
issued. Failing to limit the validity in this manner is a potential
security hole.

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[Eric2017a]

I signed the CLA

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[Eric2017a]

Just to add a little background to this change. Per the JWT specifications, the iat claim is used to determine the age of the JWT, but does not affect validity processing in any manner per section 4.1.6 of RFC 7519. Implementations that treat the iat claim as a not before time are incorrect. JWTs are valid from the beginning of time till then end of time, regardless of the issue time, unless the JWT is constrained by exp and/or nbf claims. JWTs without exp claims are valid till then end of time, while those missing nbf claims are valid from the beginning of time.

The fix here limits duration of the JWT claim to the time the claim was created till the exp claim date, if any. A better fix would be to add an option to set the not before time explicitly via a property (with a null/empty property value defaulting to the iat time and a missing property omitting the nbf claim entirely similar to the way exp is handled).

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CLAs look good, thanks!

[Eric2017a]

Fixed up author info, commit name should now be correct.