Reverse Proxy support for missmatched baseURI paths

[gav-]

It should be possible to inform RESTfm what base URI path is in use from outside a reverse proxy, which may be different to the directory that RESTfm is installed to (which is used by the report/diagnostic page to help installation problems). At worst this is a cosmetic issue with generated links and errors reported under diagnostics, otherwise RESTfm should work as expected.