use webrick as the server and daemonize only if explicitly directed to

Fixes #15.  Added daemonize option and ensure that stork only daemonizes if that option is given.  Thin was removed and replaced with webrick because the headers it prints out at the start of the thin server cannot be turned off or customized without monkey patching thin.  It would be good to see if we can make the http server choice flexible and work with any of the rack compatible servers.