converted accept_webtube into a class method of WEBrick::HTTPServer
renamed the close_explanation parameter of Webtube#close to just explanation to reduce redundancy
introduced webtube/vital-statistics.rb
introduced WEBrick::HTTPServlet::WebtubeHandler, WEBrick::HTTPServer#mount_webtube, restored accept_webtube as an instance method of WEBrick::HTTPServer instead of class method, and implemented tracking of live Webtube instances so that they could be shut down cleanly together with the server
added a sample WebSocket-only server making use of mount_webtube