Add support for downloading artifacts from matrix builds

Examples:
```
omego download --branch OMERO-5.1-latest --skipunzip matlab -v # Defaults to ICE=3.5
omego download --branch OMERO-5.1-latest --skipunzip matlab --labels=ICE=3.X -v
```
Multiple labels in any order (not much practical use at present):
```
omego download --branch OMERO-5.1-latest --skipunzip matlab --labels=label=ome-c6100-3,ICE=3.3
```
Must have only one match, otherwise should fail:
```
omego download --branch OMERO-5.1-latest --skipunzip matlab --labels='' -v
omego download --branch OMERO-5.1-latest --skipunzip matlab --labels='X=nonexistent' -v
```

Also `--dry-run` goes a bit further (downloads job XML), and an invalid job name is trapped:
```
omego download --branch OMERO-5.1-latest --skipunzip matlab -vn
omego download --branch NON-EXISTENT-JOB --skipunzip matlab -vn
```
Non-matrix jobs should still work
```
omego download --branch OMERO-4.4-merge-ice35 --skipunzip matlab -v 
```
And `omego upgrade` should work too.