worker_pids() does not return correct set of pyres worker pids

The worker_pids() method executes "ps -A -o pid,command | grep pyres_worker", which potentially returns the pid of the grep process in addition to the worker pids. When there are no workers running, it returns an array with an empty string as its only element. This does not seem to fit the expected behavior of this method. In addition, it should not really be an instance method.

I've fixed the logic for getting the list of worker pids, made it a class method, and added a test case. I've also fixed a small (unrelated) bug in the ResQ.__str__() method.