Add sample for extending EJBs

This adds another AcmeFooBean which extends FooBean. 

Shows that you can re-use an existing EJB implementation (along with its injected dependencies) by using inheritance. Sure you have to configure the injection in the application.xml now that there are two bean implementations of the same interface...

It was crucial to add `implements Foo` in `AcmeFooBean`, with the `extends FooBean` alone it would otherwise error at runtime...