Fix for #22 and #218

#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.
Not sure if you have used `git mv` for all those changes. If not please reconsider that when you re-create the patch. If you don't do this we will loose all the version history.