Initial minimal X509Certificate interfaces

This will be expanded in the future to include algorithm identifier,
subject, issuer, extensions, etc
add backend interface for loading x509 certificates
update docs with review feedback
link datetime better and add naïve to spelling list
add load_der_x509_certificate X509Backend method
add X509Certificate version attribute
Merge pull request #1442 from reaperhulk/x509-interface

X509 interfaces