Don't apply - OGM-45 Using test runner instead of superclass for SF handling

Creating this preview PR as base for discussion. The idea is to provide a JUnit 4 runner for OGM tests which does the session factory handling, without requiring tests to extend a superclass. It is used like this:

    @RunWith(OgmTestRunner.class)
    @ForEntities({ PolarBear.class, Giraffe.class })
    public class AnimalFarmTest {

        //injected by the runner, same instance for all test methods of this test
        @TestSessionFactory
        public SessionFactory sessions;

        @Test
        public void shouldCountAnimals() throws Exception {
            Session session = sessions.openSession();
            ...
            session.close();
        }

        //optional
        @SessionFactoryConfiguration
        public static void configure(Configuration cfg) {
            cfg.setProperty(
                Environment.MONGODB_ASSOCIATIONS_STORE,
                AssociationStorage.COLLECTION.name()
            );
        }
    }

Alternatively a JUnit 4 _test rule_ could be used. But this has several shortcomings:

* It can't access methods of the test class, so something like `configure()` above isn't possible. Given that we currently only use this method to set properties, one could think about an annotation based approach for setting these. One can't invoke something like `AssociationStorage.COLLECTION.name()` within an annotation, though.
* AFAICS a rule can't suppress single test methods, which we would need for `@SkipDialect` (which is apparently not used though atm.).

Based on that I think a runner is more suitable here. Any thoughts?