#732 HierarchicalContextRunner: Bugfixes and Extension Points

Modifications on the BlockJUnit4ClassRunner and the ParentRunner to allow easily extending the JUnit core without code duplication within the experimental HierarchicalContextRunner.

After I broke down all modifications, I was surprised that there are so few changes to avoid code duplication. Those changes in the BlockJUnit4ClassRunner seem more or less bugfixes to me. The annotated methods should always be obtained from the target object, otherwise the invocation will fail badly.

The modification within the ParentRunner allows to explicitly override the generation of class rules statement, which is sufficient as an extension point.

The modification within the TestClass are more or less a bit of a convenient thing, but I don't like getter-chains that much, therefore, I decided to introduce this delegator.