worker status methods and tests

### new methods
- `queue.workers(callback)`
  - returns: `{ 'host:pid': 'queue1, queue2', 'host:pid': 'queue1, queue2' }`
- `queue.workingOn(workerName, queues, callback)`
  - returns: `{"run_at":"Fri Dec 12 2014 14:01:16 GMT-0800 (PST)","queue":"test_queue","payload":{"class":"slowJob","queue":"test_queue","args":[null]},"worker":"workerA"}`
- `queue.allWorkingOn(callback)`
  - returns a hash of the results of `queue.workingOn` with the worker names as keys.