[IM] Added per @rowanthorpe

See #62
> I would just add that I found adding that 'urlbase' value to the config has proved very helpful
> for skinning (I've referenced it several times in doc pages especially), and I suggest that
> perhaps it is worth adding that to application.ini.dist anyway, irrespective of its use for cms
> login...?
Merge branch 'master' of https://github.com/inex/IXP-Manager