While looking at issue #39, it was found that shutdownNow also did not folow java.util.concurrent.ExecutorService definition.
This commit fixes the shutdownNow case, but does nothing for the shutdown() case yet.
Reduced code duplication by using the RunnableFuture inside the PriorityScheduledExecutorServiceWrapper
RunnableFuture is an implementation of java.util.concurrent.RunnableFuture
Pull this inner class out to a top level class file that is a bit more generic.  Right now it is only used here, but we may want to expand that.
Spacing change and renamed this file...not sure if I want this to be an implementation or an interface yet
Cleaned up interfaces and implementations with futures.  I also renamed RunnableFuture to RunnableFutureImp (for the time being) to avoid the confusing from java.util.concurrent.RunnableFuture
Simplified the Future implementation by not storing both the runnable and a callable, but instead converting the runnable into a callable.
Renamed these classes...I hate using "Imp" at the end of class names
Fix for checkstyle warnings after class renames and other changes