[2.2] [Form] Added form type "entity_identifier" (#1946)

(You can have a look at the issue #1946)

The "entity_id" type allow you to do things such:

```javascript
        $builder->add('city', 'entity_id', array(
            'query_builder' => function($repository, $id) {
                return $repository->createQueryBuilder('c')
                    ->where('c.id = :id AND c.available = 1')
                    ->setParameter('id', $id);
            },  
            'class' => 'Gregwar\TestBundle\Entity\City',
            'required' => false,
            'hidden' => true
        )); 
```

So, with some JS logics and UI, you can fill the field directly using the entity id.

If you pass hidden => false, a text input will be rendered, which can allow an user to provide the id manually.

If you don't pass a query_builder closure, ->find() will be used.