Custom getter/setter for BCrypt::Engine.cost

To maintain backwards compatibility for people who were manually overriding the BCrypt::Engine::DEFAULT_COST externally.

Via @tmm1 on https://github.com/codahale/bcrypt-ruby/pull/63
Merge pull request #64 from tjschuck/make_cost_globally_changeable_externally

Custom getter/setter for BCrypt::Engine.cost