Register annotations namespace

When using the standalone library, it is necessary to register the library's namespace with doctrine using the method call:

    \Doctrine\Common\Annotations\AnnotationRegistry::registerAutoloadNamespace(
        'JMS\\Serializer\\Annotation',
        "path/to/jms/serializer/"
    );

otherwise there is a fatal error.

This pull request fixes this issue by registering the autoload namespace in the builder.