Put bsb-native on opam

[bsansouci]

This is a pretty important thing for folks who want to use bsb-native and would like opam users to use their libraries.
I think right now you can't publish a project to opam using bsb-native because you wouldn't be able to build.

Here seem to be what needs to be done.

• figure out the correct META, bsbnative.install and opam files

For JS compilation

• figure out how to "install" the stdlib (everything under lib that isn't an executable)
• figure out how to get the path to the installed stdlib inside bsb-native

For native compilation

• figure out how to make bsb-native always call ocamlfind ocaml[c|opt] instead of using the vendor ocaml, and make bsb-native depend on 4.02.3+buckle-master.

For the last bullet point, fear not, I'll make another version bsb-native that supports 4.06.