Event system

Right now we have the hardcoded `beforeSave()`, `afterSave()`, etc. methods.  I'm wondering what your thoughts are on having a lightweight event system - so in your `Entity`s constructor you would do something like:

```php
$this->before('save', 'name_of_a_method');
$this->before('delete', function($mapper) { // A callable });
$this->after('loadCollection', array('Static_object', 'static_method'));
```