Write (but don't use) sourceMaps (but not sources) for client programs.

Add builder.writeToGeneratedFilename helper and use it a lot.
refactor disk layout of client programs to get rid of static/static_cacheable

This introduces some hacks into how asset dirs work. Will fix later. Might have
broken server assets. Who knows.
serve source maps (but not sources)

they don't seem to actually work in chrome yet
Serve sources as well, and use X-SourceMap header.

Source maps now work in Chrome, if you enable them (dev tools -> gears button ->
enable source maps). I can't get them to work in FF 24 though.
Comment about various browser's support for source map headers.
Fix production mode bundles.