First set of exercises for events & callbacks

Includes basic outline for Events & Callbacks page and a set of exercises on Passing Functions to Functions.

Mostly focused on practice with the syntax of passing named & anonymous functions as arguments, so it is more of a mental exercise in building the notion of functions as objects than an exercise in learning new terms or methods.
Sorry, I had forgotten that the browser and node use different classes: http://nodejs.org/api/events.html#events_emitter_addlistener_event_listener

```
var events = require('events');
eventTarget = events.EventEmitter.new();
eventTarget.addListener('woah', function() {
  console.log("I know event-fu");
});

eventTarget.emit('woah');
```

It's probably fine to do a browser example, especially if it includes the HTML/etc. to be fully runnable.