Port/gasoline

This is to add bsdowl v2.2 and gasoline to opam.

References:
- ocaml/opam#1812
- michipili/bsdowl#43
- michipili/gasoline#4
Using `<pkg>:share` isn't an ironclad requirement but each of your packages will automatically have that directory created for them and most packages defer to opam/ocamlfind for management of `share` files and `lib` files respectively (with opam understanding ocamlfind's file system layout, as well). I've created ocaml/opam#1839 to track the documentation issue.