Added scheduleAtFixedRate to SimpleSchedulerInterface
I found a way to implement this with minimal to no impact to the normal execution case.  By having this implemented in PriorityScheduler and SingleThreadScheduler, it makes their ExecutorServiceWrapper much simpler.
Some small changes to reduce the size of the jar
Merge pull request #108 from jentfoo/experimental-scheduleAtFixedRate

Added scheduleAtFixedRate to SimpleSchedulerInterface