Update README with details about expanded ListenerHelper functionality
Update README.md
This resolves #87 - Renaming PriorityScheduledExecutor into PriorityScheduler
@joshuawarner32 convinced me to go ahead with the rename (which I was leaning to slightly ahead of time, and obviously attempted once before 2.0.0).
This rename should be pretty complete, but it is completely backwards compatible.  This renames PriorityScheduledExecutor into PriorityScheduler...as well as renames PriorityScheduledExecutorServiceWrapper into PrioritySchedulerServiceWrapper.
This should be a completely backwards compatible change, the old classes still remain, and are just marked as deprecated.  They will be removed in the 3.0.0 release.