Review implementation of cryptographic random numbers

[chris-giblin]

Review, document, implement approach to generation of cryptographic random numbers.
Commenting on CryptoUtils.java, @tombentley points out in PR #9 :

Is there a reason to not use a static SecureRandom? Calling nextBytes() on a fresh instance means it will have to seed from an entropy source (I'm assuming /dev/random or /dev/urandom is used in some form or other) every time, which might end up blocking if there's not enough entropy.