KeyGenUtils API

This is the API I've written for generating Keys, nonces and ivs. It will also convert byte arrays to into SecretKeys, KeyPairs, and a couple other classes. 
It looks like taking a `ByteBuffer` would allow moving that concern away from this code, which might be preferable? http://docs.oracle.com/javase/6/docs/api/java/nio/ByteBuffer.html#wrap%28byte[],%20int,%20int%29