Pass an array of messages when creating new Validator

This PR allows you to pass in an array of custom error messages when creating a Validator instance, much like you could in L3.

While I understand you may want to encourage users to keep messages organized and promote internationalization I believe having this added flexibility is essential.

I myself have my own wrapper for validation, and was utterly disappointed when I realized there was no way (that I could see) of defining messages during run time.

So basically you can now do this.

```php
$validator = Validator::make($data, $rules, $messages);
```

Identical to L3.