Puma thread Rackup bundle
http://starzero.hatenablog.com/entry/2013/04/20/230833
http://qiita.com/kazuhiro4949/items/606c1d58afcfb06f14c4
http://qiita.com/soushiy/items/8de8dfef5183fc3b515a
https://devcenter.heroku.com/articles/deploying-rails-applications-with-the-puma-web-server
https://launchschool.com/blog/growing-your-own-web-framework-with-rack-part-1