Make KeyPair, CertificateParams and Certificate Cloneable

[nthuemmel]

Being able to clone() KeyPair, CertificateParams and Certificate would help a great deal in some situations.
Right now the only solution seems to serialize and then deserialize again

[est31]

Yeah, we should probably implement all the relevant traits listed in the API guidelines

[est31]

This depends on ring: briansmith/ring#859

[cpu]

CertificateParms is clone now. Certificate is not, but I think we'd accept a PR to make it so. I don't think it's a good idea to make KeyPair clone.

[cpu]

I think we can consider this done w/ #319.