Event handler

WIP, Do not merge. 

Goes with https://github.com/ocsigen/js_of_ocaml/pull/150

Not working yet (still an issue with `clientserver/Eliom_content.eliomi`). I haven't yet removed the bazzilions signature file that become useless with that.

I removed some Obj.magic ! <3

@balat : Just to be sure, it's safe to have covariant client_values ? In practice, it's already used as covariant for events (but through Obj.magic).