Certificate Transparency Log Support

A request to log certificates to a certificate transparency server was recently raised as an issue (https://github.com/cloudflare/cfssl/issues/318) (which has since been closed, as the feature was added).

Here are the current requirements:

• Add the ability to log issued certificates to a set of CT logs
• Add the ability to submit a pre-certificate to a set of CT logs and insert a the set of SCTs into the certificate
• CLI and API support in both sign and gencert/genkey
• Add the ability to validate SCTs