Events: Allow for easier application extension with hook points.

How does it work?

Every time someone wants to extend the application, instead of modifying original `publishing.js` file, he or she can subscribe to given events, e.g.

Scenario: Table Of Contents extension

```
// resources/public/js/toc.js

events.subscribe('content:rendered', function () {
  // Prepend table of contents in edit mode on updates
});

events.subscribe('document:loaded', function () {
  // Prepend table of contents on initial load in edit view and preview
});
```

Now, having structured way to hook into app allows to keep the main codebase maintainable and pretty stable and extremely extensible at the same time.

resources/template.html

@@ -12,5 +12,11 @@
 		%CONTENT%
 	</article>
 	%FOOTER%
+
+	<script src="/js/events.js"></script>
+	<script>
+	  events.publish('document:loaded');
+	</script>
+
 </body>
 </html>