add lwt-log lib (unix free)

This is an attempt to remove unix dependency over Lwt log engine in order to use it on the browser (with js_of_ocaml)

- New lib lwt.log : Lwt_log_core, Lwt_log_rules
- Lwt_log (with unix specifics only) still in lwt.unix

I've tried to avoid breaking existing code by keeping Lwt_log signature.

Should I use a different naming ? Any suggestions are welcome.