omego install

As pointed out by @pwalczysko the current behaviour of  `omego upgrade` automatically creating an `OMERO-CURRENT` symlink if it doesn't exist is annoying if you inadvertently run omego from the wrong directory. This PR just moves that behaviour into a new `omego install` command (optionally with `--initdb`).

`install` and `upgrade` should also check straight away for the presence/absence of the `OMERO-CURRENT` symlink

Other than this `omero install` is identical to the previous `omego upgrade` behaviour, i.e. not yet a proper installer.

In addition travis has been split into two sub-builds, the old wget+upgrade and a new install, though if you trust the install command enough it could replace the wget and setup commands, and we'd be back at a single build. The upgrade test now checks a multi-stage db upgrade works (4.4->5.1DEV)

Example commands are in .travis.yml
e.g.
`omego install --initdb --dbhost localhost --dbname omero -v http://downloads.openmicroscopy.org/omero/5.0.0-rc1/artifacts/OMERO.server-5.0.0-rc1-ice34-b10.zip`
`omego upgrade --branch=OMERO-5.1-latest --labels=ICE=3.4 --upgradedb --dbhost localhost --dbname omero -v`