Make DiscriminatorMap optional in Inheritance Mapping

It would be nice if a DiscriminatorMap was optional when defining Inheritance Mapping. For example, in dynamic environments where the actual classes that will extend a base class are not known, it would be much easier to have a default discriminator map created by Doctrine, which just mapped the class name to itself.

Having the DiscriminatorMap is great for some uses, but I think it should just use the classnames by default. That would mean less work for the developer and a more flexible/simpler system overall.