[Form] Accept a closure as label formatter in an ObjectChoiceList

Sometimes, the __toString() method is not enough to format the labels of a choices list because you want to show differents labels on differents FormType, or because you need externals informations.

This commit adds support to closure to fix this.

ie.
```php
<?php
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $localizator = new Localizator();

        $builder
            ->add('name')
            ->add('country', 'entity', array(
                'class' => 'MyProject\Bundle\DemoBundle\Entity\Country',
                'property' => function ($country) use ($localizator) {
                    return $country->getName().' ('.$localizator->translate($country->getName()).')';
                }
                )
            )
        ;
    }
```

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        |