Allow --upgradedb for omego install

[manics]

This allows a new OMERO.server to be setup using an existing older database

[pwalczysko]

Executed ~/Work/omero-setup/omego/main.py install --upgradedb --dbname omero5_1 http://downloads.op...py.org/omero/5.2.0-rc2/artifacts/OMERO.server-5.2.0-rc2-ice35-b10.zip
This downloaded the server and upgraded the DB to the desired level (matching the server). The main difficulty here (except the Postgres upgrade, which is not the topic here) the lack of doc and examples to construct the command I have executed - omego --help is still too rudimentary. Would it be possible to list at least 2-3 examples there ?

Otherwise works fine. Ready to merge.

[manics]

I still want to clean-up some of the arguments (#27)

Once that's done I'll write some proper usage docs, and we can decide whether we want to publicise it as the recommended way of installing/upgrading OMERO.server.

[joshmoore]

Merging since we now depend on this in omero-deploy.