[RFC] make it possible to persist serializeable objects in document

While handling with the `SeoMetadata` in symfony-cmf/seo-bundle, @WouterJ and i came to the conclusion that it would be nice to do have that behavior:
https://github.com/symfony-cmf/SeoBundle/blob/master/Doctrine/Phpcr/SeoMetadataListener.php

somehow as an new field type in the phpcr-odm.
This would be cool for flat value objects. Behind the scenes the phpcr-odm can serialize into an array to persist it as it is done with assoc field and back.