(PE-4308) Certificate revocation support

Add revoke, revoked?, and get-serial to API.
(PE-4308) Fingerprint support for certs and CSRs

Adds a fingerprint function to hash a cert or CSR into a hex string
(PE-4308) Add get-subject-dns-alt-names function

For getting the list of DNS alt names from a cert or CSR,
to correspond with the existing subject-dns-alt-names function
for creating the list.
Merge pull request #37 from nwolfe/pe4308-cert-revocation

(PE-4308) API support for certificate_status endpoint