private static String getMessageDigest(byte[] payload, String algorithm)
            throws NoSuchAlgorithmException {
        MessageDigest md = MessageDigest.getInstance(algorithm);
        byte[] hash = md.digest(payload);
        return BaseEncoding.base16().lowerCase().encode(hash);
    }
Create Amazon V2 signature.  Reference:
http://docs.aws.amazon.com/general/latest/gr/signature-version-2.html
src/main/java/org/gaul/s3proxy/AwsSignature.java