If thread runner lacks atomic test, use failure description

This release resolves a NullPointerException failure in RunAnnouncer.setTestFailure(Failure) that occurred whenever a failure was encountered in a @BeforeClass method. The issue was that the failure occurs in the context of the Suite runner, but the notification is generated from a BlockJUnit4ClassRunner. The solution was to use the Description to retrieve the AtomicTest.

Resolves #53