Implement support to revoke certificates and generate CRL's

Add "chef-ssl revoke" and "chef-ssl gencrl" commands to revoke and generate CRL's. This was thoroughly tested in real life, revoking certificates for the Heartbleed mess. This addresses issue https://github.com/VendaTech/chef-cookbook-ssl/issues/10. Also updated the README, bumped version to 1.2.0, added a --save option to the issue command, added two new data bags 'revoked_certificates' and 'certificate_revocation_list', among other things.

Apologies for the many commits, I messed up with one account and can't fix it short of creating a new branch and pull request. If you prefer that I can go through that process.