Added a new Detector: Hanging Executors

So, I learned the hard way that if you don't shutdown an ExecutorService (e.g. Executors.newCachedThreadPool()), the JVM won't terminate because the threads will be alive indefinitely.  

Therefor, I offer one detector for three related types of bugs - if you never call shutDown() or shutdownNow()  [borrowing code and architecture from LocalSynchronizedCollection], if you make a local ExecutorService, or, if you replace an ExecutorService field without shutting down the old one (This is really hard to track down by hand).