store full cert pem for later analysis

[danwent]

For analysis of future security incidents, it would be good if we store the full certificate PEM once for each fingerprint seen.

This would be pretty easy to add using a second table that contains two columns: fingerprint, PEM.

Note: we should move certificate data to its own database table ( #30 ) before storing additional data.

[daveschaefer]

This would also help us track whether sites are using certs that e.g. are signed by different CAs over time.