Add CommandLine argument to set the root context of the wiki

The FitNesse Serveur listen by default on the port 9123 with the context "http://host:9123/".
It is possible to change de port but not the root context for exemple to get FitNesse responding behind the URL http://host:9123/notTheRootContext/.
This is important in the case FitNesse is behind a proxy (HTTPD) and some other applications are collocated.