Use after:now with date_format validator

Hello,

In the below, the reasons for make this pull requests:
* I cannot use `after:now` or `before:now` with `date_format:Y-m-d H:i:s`
* I think the (`after`, `before`) validator should not be related with `date_format`, because its not an inputs!
* At the same time, this code is not working: 
```php
class TestModel extends Model
{
    public static $rules = array(
        'field' => 'required|after:' . date('Y-m-d H:i:s') . '|date_format:Y-m-d H:i:s'
    );
}
```

So, I remove the functions (validateAfterWithFormat, validateBeforeWithFormat)
and its work!

Thanks