Add a callback in a model for when a row is created / deleted / updated 

The way I'd see it, is something like this.

```php
<?php
class ModelUser extends ModelDatabase {
    public function init() {
        $this->on('create', function() {
            // ...
        });

        $this->on('delete', function() {
            // ...
        });

        $this->on('update', function() {
            // ...
        });
    }
}
```

The `on` method should be protected. 

There will also be a `trigger` method to trigger an event.