This resolves #91
There is a bit more included in this commit than I originally expected.  This adds "shutdownNow" to the SingleThreadScheduler.  But it also improves the logic for the normal "shutdown" call as well.
Once I wrote unit tests for the SingleThreadScheduler, I added similar unit tests to the PriorityScheduledExecutor unit tests.  This uncovered a couple other small defects/defencies in both the PriorityScheduledExecutor as well as the PrioritySchedulerStatisticTracker.

I will detail things more line by line in the pull request.