Port/gasoline

This is to add bsdowl v2.2 and gasoline to opam.

References:
- ocaml/opam#1812
- michipili/bsdowl#43
- michipili/gasoline#4
The issue is that `%{prefix}%/share/mk` is the share directory that would belong to the package `mk`. It is advisable to install shared files into the directory corresponding to the package that installs them by using the `<pkg>:share` special variable. If you want to have multiple packages use the same share directory, they should use the share directory of the base package (assuming they are dependent on each other) or of a virtual base package (so the namespace is reserved).

Additionally, it would be nice to use opam's notion of the share directory by its `<pkg>:share` construct so that if something changes that location (e.g. opam checkpoints), your script doesn't make the assumption that it knows how to traverse from the `prefix` to the package's `share`.

It is also possible to use a `.install` file which would obviate the need for this removal script. See <http://opam.ocaml.org/doc/1.1/Packaging.html#Manuallyinstallingbinariesorlibaries> for more details.