Add useful SSL diag info to json log

Largely to assess the impact of https://poodlebleed.com/ (CVE-2014-3566),
but in the process we may as well log other handy SSL information.
ssl_session_reused is not available on precise

We still use Nginx 1.4.6, ssl_session_reused does not appear until 1.5
Update CHANGELOG
Merge pull request #19 from ministryofjustice/ssl_protocol_logging

SSL nginx environment logging