Setter / Getter for Custom Messages in Validator

Expose custom message in the Validator class. This will allow for more flexibility in setting them. At the current state you can only set custom messages with the factory.

```php
$validator = Validator::make($input, $rules);
$validator->setCustomMessages(array('password.required' => 'You forgot the password... noob!', 'required' => 'You forgot :attribute!'));
```