Allow adding of custom root handlers

[psstoev]

Add new method to the registry instances - setCustomRootHandler(handler), so that we can serve custom front-end, static files, etc. That way we will lose only the default '/', which lists the metadata pages for all services, which I feel is acceptable for now, we can put on another "system" route, like /meta, or something (like we are currently handling the /client route).