Alternative server download sources, built-in unzip

Add alternative server upgrade sources.
Original (unchanged, still supported):
* Download `omego upgrade --branch XXX`
* Local directory `omego upgrade local/directory/OMERO.server`

New:
* Local file `omego upgrade /path/to/local/OMERO.server.zip`
* Remote file `omego upgrade https://downloads.openmicroscopy.org/omero/5.0.0/artifacts/OMERO.server-5.0.0-ice35-b19.zip`
* Remote file with HTTP authentication `omego upgrade --httpuser username --httppassword password https://downloads.openmicroscopy.org/omero/<non-public>/artifacts/OMERO.server-X.X.X-ice35-bXX.zip`
    - Not tested, needs .htaccess setup somewhere.

Also uses a built-in unzip instead of an external program, `--unzip and --unzipargs` options removed.