Refactor artifact logic out of the upgrade class

- Create standalone artifacts.py file
- Add DownloadCommand to download artifacts from Hudson
- Add DownloadCommand to main.py
Convert get_artifacts_list() into a classmethod
Fix flake8 warnings
Add OMERO.matlab to the list of downloadable artifacts
Add simple test for omego download functionality
Fix TestDownload tests
Allow artifacts to be downloaded without being unzipped

Also move the skzipunzip check to the UpgradeCommand
Refactor Jenkins parser options into a JenkinsParser class

JenkinsParser wraps an existing ArgumentParser object and adds an argument
group with all Jenkins related options.
Convert --skipunzip argument into a flag
Update the download test to use new --skipunzip flag
Fix skipunzip flag check in UpgradeCommand
Merge pull request #12 from sbesson/artifacts

Add command to download artifacts from Jenkins