Download candidate builds from candidates/ and not nightly/

#22: Updated to scrapper.py to return the cli options when the _MOZDOWNLOADARGCOMPLETE is in environment. Added completion scripts for bash and zsh. Updates to setup.py for the new completion scripts.
#218: Updated nightly with candidates in scrapper.py (ReleaseCandidateScrapper::candidate_build_list_regex). Updates to tests.
Hi Henrik,

I have updated the code. I will wait for you to accept the pull request for
#218. I am hoping that I can fix #22 and any other, only when the #218 is
accepted.

Thanks,
Jay
​