Implement user-facing revocation.

[jsha]

Note that this differs from the spec a bit. I moved the serial number into the POST body from the URL, because it should be integrity-protected by the JWS signature.

[coveralls]

Coverage decreased (-0.68%) to 39.96% when pulling d860ae7 on revocation-wfe into c4497ac on master.

[coveralls]

Coverage decreased (-0.88%) to 46.27% when pulling e997f00 on revocation-wfe into f6c7b49 on master.

[coveralls]

Coverage decreased (-0.86%) to 46.29% when pulling 2dec9f2 on revocation-wfe into f6c7b49 on master.

[coveralls]

Coverage decreased (-0.86%) to 46.29% when pulling eeeab2a on revocation-wfe into f6c7b49 on master.

[kuba]

Note to self: client will, obviously, have to be updated. However, we don't implement revocation at the moment anyway, so nothing would break if demo were to be updated.

[coveralls]

Coverage decreased (-1.06%) to 46.93% when pulling 048f10f on revocation-wfe into ec88d9d on master.

[jsha]

Rebased against latest master, ready for review. Also the spec fix is now merged.

[coveralls]

Coverage decreased (-0.87%) to 43.89% when pulling f8d54a7 on revocation-wfe into a20f145 on master.

[jcjones]

This looks good to me. I am merging.