Fix issue with handling of exceptions thrown by test rules

If an attached test rule threw an exception, handling of the failure resulted in a NullPointerException. This was caused by an inability to associate the failure to its corresponding "atomic" test. This release resolves the issue by mapping each BlockJUnit4ClassRunner with its associated "atomic" test much earlier in the process. I added the ability to map Description objects to "atomic" tests as well, which is why this is a feature release.