Improved secret key generation example

Improved secret key generation by randomising seed and adding SHA-2 (SHA256) hash digest of it.  Added b64 and hexdigest variants.

Should probably use random.SystemRandom(Random) instead of os.random(), but this is supposed to be a quickstart and code should have a fallback to os.random f random.SystemRandom is not available.

Uses random, hashlib, os and (optionally) base64 standard modules instead of just os.