Addition of notes-server option.

Enable to run the slides on various hosts and still control it from a
master point of view.

https://github.com/hakimel/reveal.js#server-side-speaker-notes

default.revealjs

@@ -228,6 +228,10 @@ $endif$
         dependencies: [
           { src: '$revealjs-url$/lib/js/classList.js', condition: function() { return !document.body.classList; } },
           { src: '$revealjs-url$/plugin/zoom-js/zoom.js', async: true },
+    $if(notes-server)$
+          { src: '$revealjs-url$/socket.io/socker.io.js', async: true },
+          { src: '$revealjs-url$/plugin/notes-server/client.js', async: true },
+    $endif$
           { src: '$revealjs-url$/plugin/notes/notes.js', async: true }
         ]
       });