Compiler: use findlib to load js files

js_of_ocaml +weak.js ... (use findlib to find js_of_ocaml dir)
js_of_ocaml +eliom.client/eliom_client.js ... (use findlib to find eliom.client dir)

we could then embed more js files in js_of_ocaml (ouside the runtime) (ie: unix.js)