Fix Doctrine ODM Support

Changes `Faker\ORM\Doctrine\EntityPopulator` to manage Doctrine ODM Documents.

Using the Class `Doctrine\Common\Persistence\Mapping\ClassMetadata` instead of `Doctrine\ORM\Mapping\ClassMetadata`

Managing ODM ReferenceOne as ORM One-To-One relations
(`\Doctrine\ODM\MongoDB\Mapping\ClassMetadata` does not have the `getAssociationMappings` function, I retrieve `associationMappings` using the exposed public var)

Changes generateId's signature to avoid using `Doctrine\ORM\EntityManagerInterface`; uses `Doctrine\Common\Persistence\ObjectManager` instead.