Refactored luigi.worker a bit to use multiprocessing 

...instead of os.fork and os.wait.

I thought the previous code was pretty confusing and relying on low-level system stuff. The new code makes more sense to me, but maybe it's just because I wrote it. Let me know what you think (esp @freider)

I have no idea if this works with more than 1 worker on non-Unix platforms but otoh I'm pretty shocked if the previous code was working at all.