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.