AnnotationBuilder::getFormSpecification should accept entity class names

Doctrine AnnotationBuilder::getFormSpecification should be compatible with Zend  AnnotationBuilder::getFormSpecification and should accept Entity class names as parameter.

https://github.com/zendframework/Component_ZendForm/blob/master/Annotation/AnnotationBuilder.php#L178
@param  string|object $entity Either an instance or a valid class name for an entity