Omero config

I've replaced the `--cfg config.xml` with `--replaceconfig replacementconfig.properties` and/or `--mergeconfig mergeconfig.properties` which are in a standard properties format (easier to edit or generate in scripts). This should mean `omero install` is more usable. Note that `--db*` arguments still need to be explicitly provided, they aren't read from the config file. Also note this PR is conflicting with #49 but I thought I'd open a separate PR since these are logically separate pieces of work. Alternatively see #51 for a combined PR with fixed conflicts.