add certificate document support

[carl-wallace]

No description provided.

[tarcieri]

Re: the filename, we generally prefer certificate/document.rs over certificate_document.rs when there's already a certificate.rs.

If you fix that, adding something like this is OK for now to unblock your other work, but we'll probably circle back on removing the der::Document trait in favor of a generic type before the next release. See #427 (which I intend to finish up fairly soon).

[carl-wallace]

Changed to certificate/document. Thanks.