Put bsb-native on opam

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.

Start by locating the package metadata files (`META`, `bsbnative.install`, and `opam`) and the code that installs or resolves the JavaScript stdlib under `lib`. Then trace native compilation to see how bsb-native invokes OCaml and how its compiler dependency is selected. Done means an opam-installed bsb-native can build both JavaScript and native projects using the expected stdlib and compiler.

Written by the indexing model from the issue text.