Blake2b support

This provides wrappers to the Blake2b hash function, based on the SHA3-finalist
hash function Blake. This hash function provides speeds faster than MD5 in
software but with the security guarantees required of all SHA3 candidates.

Blake2b is conditionally provided if the libsodium version is 0.4.0 or greater.
Merge pull request #56 from cryptosphere/blake2b

Blake2b support