Implement "bcrypt+SHA256" as default algorithm (independent to straight bcrypt).

[boughtonp]

The straight implementation of bcrypt only uses the first 72 characters of a passphraase. Whilst this is sufficient for most needs, it would also make sense to pre-hash with SHA-2 to use the full passphraase, and make this the default option. (The straight bcrypt option would remain available for compatibility with other implementations).