Rakefile: add build and test tasks.

Rakefile

@@ -0,0 +1,17 @@
+require "rake"
+
+task :default => :build
+
+desc "Build the site."
+task :build do
+  sh "bundle exec jekyll build"
+end
+
+desc "Run html proofer to validate the HTML output."
+task :test => :build do
+  require "html-proofer"
+  HTMLProofer.check_directory("./_site",
+      :parallel => { :in_threads => 4 },
+      :favicon => true
+  ).run
+end