Added support for custom violation messages on the 'required' validation constraint

Should be defined like so:

```php
'value' => array(
        	'type' => 'string',
        	'required' => array(
                'value' => true,
		  'message' => 'Custom message goes here'
            ),
        ),
```

'value' property also takes functions and strings.
Smushed the logic a bit to reduce the repetition.
Merge pull request #6 from wave-framework/validator-custom-violation-messages

Added support for custom violation messages on the 'required' validation...