Add BCrypt support to sprest-security password salting

[markschaake]

See: https://github.com/t3hnar/scala-bcrypt for scala impl

See: http://www.mindrot.org/projects/jBCrypt/ for java library that above uses

What's really cool about BCrypt is that it embeds the salt into the hash and is able to extract the salt from a given hash. Allows you to forget about the salt (i.e. no need to store it separately or have a strategy for combining / extracting salt+hash)