Added the possibility to define custom attributes's names on models

Added the possibility to define the last argument of [Illuminate\Validation\Validator](https://github.com/laravel/framework/blob/master/src/Illuminate/Validation/Validator.php#L140) constructor method.
Where's this variable coming from? `$customMessages` is an argument but the new one is not. Please include it in the method signatures if it makes sense (I guess it does although I'm no Laravel expert) or simply remove this check altogether and use `static::$customAttributes` directly.