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>