[fix #155] Avoid overwriting configuration

When the installation script is invoked, it first checks that the places
where files are about to be installed are free. If directories already
exist, just abort installation.
[imp #155] Add installation upgrade script

This is a quite simple script which won't handle all cases but should be
able to upgrade from installations made with the provided `install.sh`
script.
Merge pull request #167 from satabin/feature/upgrade-script

Feature/upgrade script