Add Doctrine ORM serialization form type

Here is a `DoctrineORMSerializationType` form type.

This form type reads `JMSSerializer` serialization class metadata and uses `Doctrine` ORM entity metadata
to generate form fields and correct types.

All you have to do is to define a form type service for each entity for which you want to use a form type, like this:

```xml
<service id="my.custom.form.type.post" class="Sonata\CoreBundle\Form\Type\DoctrineORMSerializationType">
    <tag name="form.type" alias="my_custom_form_type_comment" />

    <argument type="service" id="jms_serializer.metadata_factory" />
    <argument type="service" id="doctrine.orm.default_entity_manager" />
    <argument>my_custom_form_type_comment</argument>
    <argument>My\CustomBundle\Entity\Comment</argument>
    <argument>a_serialization_group</argument>
</service>
```

The service definition should contain the following arguments:

* The JMSSerializer metadata factory,
* The Doctrine ORM entity manager,
* The form type name,
* The entity class name for which you want to build form,
* The serialization group you want serialization fields have.