Properities should not be required for object as elastica can automap
Merge remote-tracking branch 'peerj/ObjectWithoutProperties' into serializer-integration

Conflicts:
	Transformer/ModelToElasticaAutoTransformer.php
Replace deprecated calls `Document::add` to `Document::set`
Allow empty or null or no `mappings:` key under type configuration

refs #300. This commit allow to define types without having to
set any mapping as Elasticsearch build his own.

The minimal config become:

    indexes:
        toto:
            client: default
            types:
                Article:
                    mappings: ~
                    ...
Ref #341, fix the insertMany method to use Bulk indexing

To make it works, I inject the serializer defined for the Type
into the fos_elastica.object_serializer_persister service.

This is the SAME service injected in the setSerializer of Type.

We deport the handling of serialization outside Elastica,
this is not so good but we need to build our own Documents to
get the ID's correctly.