fix 'Invalid date string provided' because of wrong \DateTime formatting

When submitting french formatted dates with a Date form element and so, a custom format (d/m/Y), DateTimeFormatter fails because of wrong date formatting.

example :
```php
$this->add(
            array(
                'name'       => 'mydate',
                'type'        => 'date',
                'attributes' => array(
                    'id'    => 'mydate',
                ),
                'options'    => array(
                    'format'=>'d/m/Y'
                ),
            )
        );
```

Trying to validate 31/01/2013 fails with 'Invalid date string provided' 

```php
$value->format($this->format);
```
When the value is a string, we must consider using DateTime::CreateFromFormat() to format the date accordingly to the format that is set in the form element.
When the value is an int (timestamp), the format ->format() method can be used