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.
Part of me feels like this doesn't "do enough" to get people to think through what's happening. How about:

```
var animalToCatTranslator = function(sentence) {
  return sentence.gsub('animal', 'cat');
}

var animalToDogTranslator = function(sentence) {
  return sentence.gsub('animal', 'dog');
}

var favoriteAnimal = function(animalTranslator) {
  return animalTranslator("animals are the best!");
}

favoriteAnimal(__)
// Should evaluate to "cats are the best"

favoriteAnimal(__)
// Should evaluate to "dogs are the best"

// Search suggestion: Call a javascript function with another function as the argument
```
This gives them all the core pieces and provides a core which could be built on top of like:

* Have them write their own translator (say for emus)
* Have them write their own user for a translator (say, `leastFavoriteAnimal`)