Complete _oasis to synchronized it with the OPAM package

[Chris00]

This allows to generate the OPAM files with oasis2opam — the dev version (to be released soon). Some dependencies that I added may not be necessary (but they were present in OPAM..., feel free to remove them and regenerate the OPAM file — another reason why automation is nice!).

Note that the license in _oasis and OPAM differ.

[hannesm]

thanks