Add a favorite button to stories

This adds a button to favorite a story. Popular stories are now sorted by the number of favorites they have.

Model classes also have support change notifications now. Parse doesn't support key value observing, so I had to implement my own change system.

Example usage

```swift
var user = User()

// Add a subscription
var subscription = user.subscribeToChanges() { (key, value) -> Void in
  // do something
}

// Remove the subscription
subscription.cancel()
```