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/edit.html

@@ -8,6 +8,7 @@
   <link href="/style.css" rel="stylesheet" type="text/css" />
   <script src="//cdnjs.cloudflare.com/ajax/libs/marked/0.3.5/marked.min.js" type="text/javascript"></script>
   <script src="//cdnjs.cloudflare.com/ajax/libs/blueimp-md5/1.0.1/js/md5.min.js" type="text/javascript"></script>
+  <script src="/js/events.js" type="text/javascript"></script>
   <script src="/js/publishing.js" type="text/javascript"></script>
   <base target="_blank">
 </head>