add displayabled values support

some times we need custom displayable values, especially when using multi-byte encoded text scenes. for example:

HTML:
```html
<label>banner type</label>
<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>
...
<div>
    <label>banner image</label>
    <input type="url" name="image" value="" placeholder="the url of banner image.">
</div>
<div>
    <label>banner video</label>
    <input type="url" name="video" value="" placeholder="the url of banner video.">
</div>
...
```

Rule:
```php
    ...
    $rules = array(
              'content_type' => 'required|in:1,3,4',
              'image'        => 'required_if:content_type,1',
              'video'        => 'required_if:content_type,3',
             ),
    ...
```

app/lang/en/validation.php:

```php
    ...
    'attributes' => array(
                     'content_type' => 'banner type',
                     'image'        => 'banner image url',
                     'video'        => 'banner video url',
                    ),
    ...
```

Request:
```php
array(
 'content_type' => 1,
 'image'        => '',
);
```

Errors:
> The banner image url field is required when content type is 1. 

Difficult for users to understand what the number `1` is 

We expect that this:

> The banner image url field is required when banner type is Image.

Now we can add the `values` ​field to the `app/lang/en/validation.php` to achieve it：
```php
    ...
    'values' => array(
                 'content_type' => array(
                                    '1' => 'Image',
                                    '3' => 'Video',
                                    '4' => 'Web'
                                   ),
                ),
    ...
```

Thanks!

There's no method to set `$customValues` like there is to set `$customAttributes` (unless one extends Validator class):

Custom attributes may be set either in constructor or using `setAttributeNames(array $attributes)`.
I think this cool functionality deserves similar API.

To explain: I'm not using the default key validation.values because my models attributes overlap.