Introducing PasswordHash

By @fraga, replaces PR #49 - consolidates all the commits into one. Per
@fraga there's a false negative that still needs to be resolved.
Rework

This changes crypto_pwhash_scryptxsalsa208sha256 to
crypto_pwhash_scryptxsalsa208sha256_str, simplifies the interface, etc.
Update documentation
Merge pull request #51 from adamcaudill/fraga_ImplementCryptoPwHash

Introducing PasswordHash