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.