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.
In terms of our tests, we have those to verify that download will succeed. We are aware of that test data will not always be up-to-date. Means we don't want to update the version numbers each time we test those files. It doesn't matter for us, if it is Firefox 10.0, 21.0, or 28.0. Important is that we download the correct version. Only in case that the structure on the FTP server is changing, we will have to make sure to update those versions. So please only move the test files to the new location, and update the target_url in terms of the `nightly` -> `candidates` change.