Provide shutdown hook for story cancellation to clean up e.g. open Firefox

When the running time of stories exceeds storyTimeoutInSeconds, the Futures get cancelled without cleanup the resources, e.g. the Firefox stays open even though the jvm running the tests stops.

This is especially bad if the tests are run via a cronjob since the firefox processes accumulate over time slowing down the machine.

I'm not quite sure how to implement this, the canceled Futures get a CancellationException, to shut down Firefox I think it is necessary to run the @AfterStory or @AfterStories annotated methods plus it would be good to put a stack trace in the report which currently doesn't show up.