Review of manager system

Override of EntityManager to implements getManager() via EntityManagerFactory.

so now: 

`$em->getManager('UserBundle:User');`

and via services : 

    hydra.user_bundle.user_entity_manager:
        class: Hydra\UserBundle\Manager\Entity\UserEntityManager
        factory_service: 'doctrine.orm.default_entity_manager'
        factory_method: 'getManager'
        arguments: [ %user_entity% ]
        
with : 

    parameters:
        user_entity: 'UserBundle:User'
    
    services:
        hydra.user_bundle.user_repository:
            class: Hydra\UserBundle\Repository\UserRepository
            factory_service: 'doctrine.orm.default_entity_manager'
            factory_method: 'getRepository'
            arguments: [ %user_entity% ]