Fixed a bug that prevented Model::saving() from being extended

Laravel's event halting system prevents certain methods from cascading when any type of non-null content is returned. This is the case for the following methods:

* deleting
* saving
* creating
* updating

Since Model::boot() registers the `saving` event handler and *always* returns either `true` or `false`, the `saving` event is effectively rendered useless for any code that wants to implement this on models that extend `\Way\Database\Model`.

*Proposed solution:* the `saving` event should *only* return *false* on validation errors and should return *null* on succesful validation.