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.
Merge branch 'unstable_staging' into more_javadocs+comments
Added more javadocs to protected and private functions.
The goal is to add comments in general to help make it easier for others to follow the code and understand what is going on where.
I cleaned up a few minor things that I saw as I went through the code looking for additional places to add comments.
Merge pull request #92 from jentfoo/issue91_SingleThreadScheduler_shutdownNow

Issue91 SingleThreadScheduler shutdownNow functionality (plus bug fixes)
Merge branch 'more_javadocs' into unstable_staging