Return references from load()

Currently, it’s impossible to retrieve persisted objects by their reference name in tests. This PR returns the references (YAML keys) from the `load()` method, so one can use AliceBundle with [DoctrineDataFixtures’ `SharedFixtureInterface`](http://symfony.com/doc/2.0/bundles/DoctrineFixturesBundle/index.html#sharing-objects-between-fixtures):

```php
class LoadDevData extends DataFixtureLoader implements SharedFixtureInterface
{
    private $referenceRepository;

    public function load(ObjectManager $manager)
    {
        $references = parent::load($manager);
        foreach ($references as $name => $object) {
            $this->referenceRepository->add($name, $object);
        }
    }

    public function setReferenceRepository(ReferenceRepository $referenceRepository)
    {
        $this->referenceRepository = $referenceRepository;
    }
}
```

And then in a test:

```php
$referenceRepository->get('some_yaml_key');
```

Additionally, I could add a  `Loader::getReferences()`. What do you think?