Hello World in Opal!

<!doctype html>
<html>
<head>
  <script src="https://github.com/opal/opal/releases/download/v0.5.3/opal.js"></script>
  <script src="https://github.com/opal/opal/releases/download/v0.5.3/opal-parser.js"></script>

  <script type="text/ruby">
    puts "Hello world!"
  </script>
</head>
</html>