[RFC] added ability to use custom persisters

When using the FOSUserBundle, you are required to use the `UserManager` to persist users.  This causes an issue for this library.

This solution is a little hackish but I think it gets my requirement across.

### Usage

Create a custom persister class:

```php
class UserPersister implements PersisterInterface
{
    protected $manager;

    public function __construct(UserManager $manager)
    {
        $this->manager = $manager;
    }

    public function persist($object)
    {
        $this->manager->updateUser($object);
    }
}
```

Add `_persister` key to your fixture file:

```yaml
MyApp\UserBundle\Entity\User:
    user0:
        username: kevin
        plainPassword: pass
        email: kevinbond@gmail.com
        enabled: true
        _persister: user
```

Load fixtures:

```php
$userManager = $container->get('fos_user.user_manager');

$userPersister = new \MyBundle\Alice\UserPersister($userManager);

// add data fixtures
\Nelmio\Alice\Fixtures::load(array(
    $fixtureDir.'/users.yml'
    ),
    $em,
    array(),
    array('user' => $userPersister)
);
```

Thoughts?