Merge pull request #17 from glebm/patch-1

Document format option in Readme
batch pool
.travis.yml: remove obsolete 1.8.7, unmaintained 1.9.2, and add jruby
fix
only spin up threads if necessary and concurrency > 1
automatically select min pool size
refactor fix repetition
enumerable#size is only available from 2.0
make request_translations private
ensure the pool is always shutdown
concurrency spec