Serializer support in providers and mapping configuration fixes

Following [https://github.com/FriendsOfSymfony/FOSElasticaBundle/pull/341#discussion_r7997040] here is a PR merging #341 and #300, and fixing #332.

As we can define the Serializer I also allowed empty `mapping` for types so we can only define a persistence provider and :boom:: READY TO GO:

```yml
fos_elastica:
    clients:
        default: { host: localhost, port: 9200 }
    serializer:
        callback_class: FOS\ElasticaBundle\Serializer\Callback
        serializer: serializer
    indexes:
        toto:
            client: default
            types:
                Article:
                    #mappings: ~ or [] or {}, whatever
                    persistence:
                        driver: orm # orm, mongodb, propel are available
                        model:  Toto\Bundle\CoreBundle\Entity\Article
                        provider: ~
```

Elasticsearch does not need any mapping to works, it built his own.

I also fixed the bulk indexing from #341, making use of the serializer callback in `ObjectSerializerPersister` directly.

Ping @lsmith77 and @leahaense.