acme: add Client.RevokeCert for certificate revocation

This change adds RevokeCert method to Client, implemented as described
in https://tools.ietf.org/html/draft-ietf-acme-acme-02#section-6.6
and https://tools.ietf.org/html/draft-ietf-acme-acme-03#section-6.5.

Change-Id: Ib0defbc299b46268c0f36dbe0f29f0081a3598d5
Reviewed-on: https://go-review.googlesource.com/27092
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
3 files changed