wish: add support for x509 certificates in text form

[nahuel]

Some software like Nginx accepts x509 certificates in text form, like the one you get running openssl x509 -in /etc/ssl/certs/ssl-cert-snakeoil.pem -text. Is simply the same PEM text but prepended by a human readable chunk describing the cert fields. It will be nice to make parseCertificate(...) detect and parse this format by extracting the PEM at the bottom and discarding all the (redundant) info at top, instead of raising CertificateParseError as currently does.