Fixed #20138 -- Added BCryptSHA256PasswordHasher

BCryptSHA256PasswordHasher pre-hashes the users password using
SHA256 to prevent the 72 byte truncation inherient in the BCrypt
algorithm.
Add Donald Stufft to the AUTHORS file
Merge pull request #958 from dstufft/prehash-bcrypt

Fixed #20138 -- Added BCryptSHA256PasswordHasher