Store cert fields in CertDB

I'm not 100% sure if this is the correct approach, so I didn't pull out all fields which are in CertificateDescription constructor. If everything is OK, then I'll add that (here or in follow up pull request). Also which fields other than those should we store? It should be easy to write a script which repopulates database with added fields, so we can add them as we need them.
Also test feels slightly pointless, because it almost copies the code in from_values, but at least we know that there are no obvious errors in description.
If there is some funny certificate, that integer could be huuuuuuuuuuge (like serial number).  And in SQLite there is a limit on size of an integer.