#9 default to a more secure default_md

[Peter Meyer]

phpki is configured to use md5 as default hashing algorithm for certificates. Unfortunately, this prompts newer Chrome versions to tag this certificate as insecure. See http://blog.davidchristiansen.com/2012/04/solved-chrome-v18-self-signed-certs-and-signed-using-a-weak-signature-algorithm/ for details.

I have confirmed, that Chrome 23 does accept certificates generated by using sha256 hash.

I suggest to change all occurences of "default_md = md" to "default_md = sha256" or stronger.