Add docs for new BCrypt::Engine.cost attribute

Shame on me for not adding docs in the first place.

See https://github.com/codahale/bcrypt-ruby/pull/63 and https://github.com/codahale/bcrypt-ruby/pull/64 for the feature implementation documented here.

/cc @tmm1

(Fun fact: submitting this from 30,000 feet up in the air!  Internet on airplanes!  The future!)