Add support of `All` constraint

This PR aims to add support of [`All` constraint](http://symfony.com/doc/master/reference/constraints/All.html) that allow to validate an array of objects.

For instance, the code bellow validate that `item` field is an array of `Club` objects.
```php
class ClubListOutput extends ListOutput
{
    /**
     * @Assert\Type(type="array")
     * @Assert\All({
     *     @Assert\Type(type="CoopTilleuls\Geophyle\AgendaBundle\Entity\Club")
     * })
     * @var array
     */
    public $items;
```
With that fix, array type will be displayed and subtype fields.
![nelmio-all-constraint](https://f.cloud.github.com/assets/804625/1491441/c0741530-47ae-11e3-908f-7f6f3b601c25.png)