improving the cert ontology

The cert ontology currently provides certificates for

http://www.w3.org/ns/auth/cert.n3

- X.509
- GPG

For the following key types

- RSA
- DSA

I am unsure that all the terms are complete for either RSA or GPG pub/priv
keys

Modulus
Exponent

Are covered, but I think there's a few more terms that are fundamental to
these keys.

We should add the terms to create a full ontology, as long as they are non
breaking (ie add new terms, dont change existing)

I'd like to also consider adding commonly used serializations.

In terms of keys it would be good to see ECC added to the list, and also
openssh supported to a degree.

I think the first step should be to complete the RSA key ontology.  Any
objections?