Make the client dir independent from the server dir.

`build.js` has been moved from `client/` to `server/`. Although it
operated on client files, it is actually part of the operation of
the server.

`build.js` now uses the `conf.public_http` setting to locate and
place its input and output files, and now honours the `-c` cmdline
flag for setting the configuration file location like `kiwi.js`
does.