Improve handling of unexpected exceptions in topics:
 * Treat exceptions in topics as failing tests and conversely treat topics as passing otherwise. Count these as honored/broken vows in the final counts.
 * Run teardown functions for a context if the topic passes, regardless of the outcome of subcontexts.
Report topic pass/fail as a testcase in xunit
Merge pull request #108 from coreypobrien/better-unexpected-topic-exceptions

Better unexpected topic exceptions