Support not waiting on job completion

[mgmarino]

Sometimes, it is useful to not wait for an mrjob to complete. An important use case is to spin up an EMR job from e.g. an AWS lambda function.

In our local code (based upon mrjob), we have developed functionality to do this. I would like to try to integrate this functionality back into mrjob as I believe it will be generally useful for the community.

[coyotemarin]

Great, that's something I've been wanting for a long time.