Parse CertificateRequest messages

The RFC has two definitions of CertificateRequest. http://tools.ietf.org/html/rfc5246#section-7.4.4 and http://tools.ietf.org/html/rfc5246#appendix-A.4.2, but turns out the one with `supported_signature_algorithms` is the correct one, says http://www.rfc-editor.org/errata_search.php?rfc=5246
Done!