Modified worker_pids() to return the correct set of pids for pyres_worker processes and made it a class method.
Added test case for worker_pids().
Fixed small bug in ResQ __str__() method.
Fixed worker_pids test case.
Merge pull request #122 from lcwill/worker-pids-fix

worker_pids() does not return correct set of pyres worker pids
Merged from upstream.