Moved rack app into its own class and moved server logic into executable.

Testing the server was impossible because of the thread and it didn't add
anything to what Rack::Handler::WEBrick was doing anyway.

The interesting stuff, which can be tested, is the rack app which is now in its
own class.
Tested the rack app a bit. Need a name for it...
RBX in 1.9 mode seems unreliable