Second tentative use of oasis and ocamlbuild.

See https://github.com/ocsigen/tyxml/pull/8

It needs testing, I'm not sure everything is exported correctly. Eliom/ocsigenserver and all my personal projects compile under that, but I didn't test the syntax extension, for example.
I'm not super convinced that the way we export the stuff is great, but that's for another day.

The documentation for Pa_tyxml is not working anymore, I don't really know how to fix it since it uses a pack.

About whether to try to include this in the release : except if it's really well tested, I would say no. But in the case we don't include this, we need to drop support of ocamlduce from the current makefile, since I don't think we want to have it anymore.