Add script to view the github pages locally

We need a development webserver to view the calculator locally,
because it does AJAX requests and those don't work over the file:// protocol.

This repo is just a collection of static files,
so there was no platform yet that could act as a webserver.
I've decided to go for ruby because that is what github pages uses
if you want more than just static files.
https://help.github.com/articles/using-jekyll-as-a-static-site-generator-with-github-pages/

serve-github-pages.rb

@@ -0,0 +1,14 @@
+#!/usr/bin/env ruby
+
+# Preview the rendered GitHub Pages in a browser.
+# Inspired by http://asciidoctor.org/docs/user-manual/#external-preview
+
+# Utilities to replace common UNIX commands in Makefiles etc
+# https://github.com/ruby/ruby/blob/trunk/lib/un.rb
+require 'un'
+
+# Pass these values as command line arguments to httpd.
+ARGV.replace [File.expand_path('.', __dir__), '--port=8080']
+
+# Start the http server.
+httpd