Renamed PriorityScheduledExecutor into PriorityScheduler
This is more consistent, and more concise.
Revert "Renamed PriorityScheduledExecutor into PriorityScheduler"

Since we did not include this in 1.3.0, and since PriorityScheduledExecutor is closer to java.util.concurrent.ScheduledThreadPoolExecutor, we will just keep the name for now.  I filed #87 to consider this further.

This reverts commit c7c27eafa9f02c1cf652bfaffae8c65f5312b3a1.