Add slide tracker

And other hacks for video recording.
This should be carefully backported to master.

slides/workshop.html

@@ -31,5 +31,24 @@
         excludedClasses: [@@EXCLUDE@@]
       });
     </script>
+    
+    <script src="/socket.io/socket.io.js">
+    </script>
+    <script>
+    var socket = io();
+    var leader = true;
+    slideshow.on('showSlide', function (slide) {
+      if (leader) {
+        var n = slide.getSlideIndex()+1;
+        socket.emit('slide change', n);
+      }
+    });
+    socket.on('slide change', function (n) {
+      leader = false;
+      slideshow.gotoSlide(n);
+      leader = true;
+    });
+    </script>
+
   </body>
 </html>