Enables ClassRules again (Fixes Issue #35)

This fixes Issue #35, by not overriding `classBlock`, but `withBeforeClasses` that way when inheriting from the JUnit 4.9 `BlockJUnit4ClassRunner` the ClassRules are still executed.
The behavior, however, is a bit inconsistent now, because `@BeforeClass` methods are execute before the dataproviders and `@ClassRule` is executed after the dataproviders.