added a periodic worker which will call a function periodically based on an input duration
review feedback. Don't use time.Tick and improve the tests
split new and run functions, add tests for calls to Stop
improved the kill test
implemented ErrKilled for signaling that the call was asked to stop
Merge pull request #469 from mattyw/add-periodic-worker

added a periodic worker

The periodic worker will call a function periodically based on an input duration