[Requrest] add displayable values support

some times we need coustom displayable values, for example:

HTML:
```html
<select name="content[type]" >
      <option value="1" data-type="image">Image</option>
     <option value="3" data-type="video">video</option>
     <option value="4" data-type="web">Web</option>
     </select>
```

Rule:
```php
...
content.type => 'required|in:1,3,4',
content.image => 'required_if:content.type,1',
...
```

app/lang/en/validation.php:

```php
    'attributes' => array(
            'content'       => array(
                                         'type'  => 'content ype',
                                         'image' => 'image url',
                                         ),
     ),
```


Errors:
The _image url_ field is required when _content type_ is 1.

Except:

The _image url_ field is required when _content type_ is _image_.