The code is in a Gist below. All files go in a single folder (say,
~/git/myproject). To run it (after
bundle installing), open two Terminal tabs in this folder. In the first one, run
guard. In the other, run
ruby serve.rb. Then hit up http://localhost:4567 in your browser of choice. LiveReload should automatically reload the page when you change
index.html (as configured; modify the Guardfile to watch additional files for changes).
(These instructions are a little vague, because if you can't easily fill in the blanks you should probably just use the app. It's not worth learning all about Ruby/Gems/Rack/Guard to save $10.)