Add CertCreator

CertCreator generates client or server public/private RSA keypairs signed by
a generated self-signed certificate authority (CA). It will reload the CA
cert from disk if present, and will not overwrite a keypair if either the
key or cert exists on disk, so a cert generation program can be safely
re-run after adding new certs. All keys are granted the minimal set of
extended key usages for their purpose.