Add an official way of getting back the result of a job

[achauve]

I use custom code for now:

job = Job(job_id)
result = job.collection.find_one({"_id": ObjectId(self._job_id)},
                                                  fields={'result': 1, 'status': 1})

but obviously it could be encapsulated in the Job class.

I looked at the Job code but the constructor parameters are not crystal clear (indeed fetch does not fetch all the job data, especially not the result). Maybe a Job(id, fetch=true) then job.result would do the job? ;-)