add support for the crit header

per http://tools.ietf.org/html/rfc7515#section-4.1.11 & http://tools.ietf.org/html/rfc7516#section-4.1.13

On the consuming side probably to fail if unknown headers are named in crit but with some way for the application to say it will handle certain named headers.

On the producing side check to see that setting an array valued header isn't too much of a PITA.

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