Added support for getting documents in unified Doctrine Listener

Commit 1d700261abb7b7f39c56550cac0732e807240b35 unified a single Doctrine Listener class, but only has support for the ORM version of LifecycleEventArgs by explicitly calling getEntity. This PR adds support for the doctrine ODM LifecycleEventArgs. Using mongodb-odm results in an error in the Listener class pre/post event methods:

FatalErrorException: Error: Call to undefined method Doctrine\ODM\MongoDB\Event\LifecycleEventArgs::getEntity() in /my/app/vendor/friendsofsymfony/elastica-bundle/FOS/ElasticaBundle/Doctrine/Listener.php line 209