Port/gasoline

This is to add bsdowl v2.2 and gasoline to opam.

References:
- ocaml/opam#1812
- michipili/bsdowl#43
- michipili/gasoline#4
Thank you for your very clear explanations. I am quite surprised by all of this because I read nothing about this in the [packaging guide](http://opam.ocaml.org/doc/1.1/Packaging.html).

How strong is the requirement on the use of `<pkg>:share` ?   I added michipili/bsdowl#142 to my backlog.