UriService and Uri changeover infra

I've created overloaded methods in core classes (primarily, listing and webagent) to allow for passing Uris around in place of strings. I've also added a UriService class with the nonworking Uris commented out for now. I'm trying to come up with the best way to do the equivalent of string.format on those. Advice?

Please look over this one. I'm working toward some core stuff and would really appreciate a second set of eyes. Thanks!