Allowing overriding of Mapping\ClassMetaData

When overriding class ClassMetadata for making customized mapping, its now not possible to use own reflection implementation, because ClassMetadata and ClassMetadataInfo implementation creates reflection classes directly.

I'm proposing a patch, which will allow for for a class overloading ClassMetadata to create it's own reflection.

The code needed to do so would be:
class MySuperClassMetadata extends \Doctrine\ORM\Mapping\ClassMetadata {
... my code

public function getReflectionClass() {
if(!$this->reflClass)

return $this->reflClass;
}

public function _getNewReflectionProperty($class, $property)

}

Which also much simplifies code from this article: http://www.doctrine-project.org/blog/your-own-orm-doctrine2

(Hope it's OK, I'm not much familiar with contributing to open source projects yet)