Remove dependencies on Thin, so Dashing can run on JRuby.

Here's a quick pass to remove the dependency on Thin (and EventMachine) in favor of Puma. This allows dashing to work on JRuby, and Puma is a great server.

I was confused about the connection-closing block that used Thin-specific APIs. We can discuss what that code is doing and how to do it under Puma.
There is no sense on put that line on the commit of your PR.
Because if it would merged, you would change the gemspec of the entire project to dashing-jruby.
I think you just want to fork that project, and on your fork only, put these line.