Make separate "CreateCertificate" and "CreateSelfSignedCertificate"
functions to take the two roles of NewCert. These names should help
clarify that they actually make certificates and not just allocate new
"Cert" or "Certificate" objects.
Secondly, in the case of non-self-signed certs, require a public and a
private key to be passed in instead of two private keys, because it's
pretty hard to tell when reading code which one is meant to be the
signer and which one is the signee. With a public and private key, you
know.
(These are some changes I made in the course of the openssl port,
because the NewCert function kept being confusing to me. It's possible
I'm just being ridiculous, and this doesn't help improve readability for
anyone else, but if I'm not being ridiculous let's get this in)
