Add command to download artifacts from Jenkins

This PR adds a new `download` command to `omego` to download artifacts from the CI server which should be used as follows
```
omego download -h
omego download server
omego download server --skipunzip true
omego download win --branch OMERO-merge-stable
```

The major changes in this PR are the following
- the artifact logic (class and command) is moved to an `artifacts.py` (since the `download` command did not fit with `upgrade.py`)
- corresponding tests are added and built as part of the Travis build
- `Artifacts.download()` does not fail if `--skipunzip true` is false. The `Stop()` is moved to the `UpgradeCommand` instead

More generally, should we keep maintaining our own `Artifacts` class or start using a 3d party pip-packages e.g. `jenkinsapi` which already has all this logic in place including MD5 checksumming etc... /cc @manics, @joshmoore