With this patch we can now do things like verify a x509 cert against a x509_ca
then extract its pubkey and test a message signature.

    cert = crypto.x509_cert()
    cert:from_pem(server_cert)
    kpub = cert:pubkey()
    message = 'Hello world'
    verified = crypto.verify('md5', message, signature, kpub)

add docs for the x509_cert and x509_ca objects.

Detect what type of key we are dealing with and use the right to_pem
function. This way we match the enconding that the openssl dsa|rsa apps
use for certificates.

check the return codes of wrote_bio_PrivateKey for any error.