Add SHA-512-256 to list of algorithms for digest passwords and SASL digest

RFC 7616 was released, indicating that SHA-512-256 is a supported algorithm for digest HTTP mechanisms. We need support for that algorithm as a password. We also then should add a corresponding SASL variant to the DIGEST suite.

There is no MessageDigest for SHA-512-256. This can be accomplished by truncating the output of SHA-512 to 256 bits (32 bytes). A wrapper MessageDigest could be created, or the ability to arbitrarily truncate the digest could be added to the DIGEST mechanism.