Make the packaging less crappy.

This removes the weird makefile (oasis can generate one that is pretty
much equivalent but less broken, and you can add things after `# OASIS_STOP` if you want), add a correct opam (that can be used
for pinning and should be used for the next release).

I also added a script to ease release (by generating the whole oasis
boilerplate, which allows the release to remain _oasis independent).