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.
Not sure how I feel about having a series of steps before we get to a "should". It seems to be a bit difficult to figure out what exactly is wanted.

Perhaps something like:

```
function conservativeSpender(balance)  {
  return balance > 100
}

function liberalSpender(balance) {
  return balance > 10
}

function shouldIBuyThis?(balance, spendingStrategy) {
  if (spendingStrategy(balance) {
    return "Sure! I've got the money!";
  } else {
    return "Nope! Gotta keep my savings up!"
  }
}

shouldIBuyThis(20, __);
// Should evaluate to "Nope! Gotta keep my savings up!"

 shouldIBuyThis(20, __);
// Should evaluate to "Sure! I've got the money!"

shouldIBuyThis(__, function superFrugal(balance) {
  return balance > 1000
});
// Should evaluate to "Nope! Gotta keep my savings up!"

shouldIBuyThis(__, function horribleSaver(balance) {
  return balance > 0
});
// Should evaluate to "Sure! I've got the money!"

function smarterShouldIBuyThis(cost, balance, strategy) {
   if (__(balance - cost) {
    return "Sure! I've got the money!";
  } else {
    return "Nope! Gotta keep my savings up!"
  }
}

smarterShouldIBuyThis(50, 45, horribleSaver);
// Should evaluate to "Nope! Gotta keep my savings up!"

```