Switch Digest Signing over to sha256 from md5

[RocketJas]

As the MD5 hash function is considered cryptographically broken, I have switched over the signing to use sha256. I have made this change to a fork and I was wondering if this was appropriate to upstream. RocketJas#1

[kornelski]

Thanks for the suggestion. I've implemented it without OpenSSL, which is a heavy dependency.

[RocketJas]

Thank you for fixing this so quickly.