Implement PG'OCaml backend

[rgrinberg]

https://github.com/darioteixeira/pgocaml/ is 99% pure OCaml (with the remaining 1% to be done very soon)

[seliopou]

Generally speaking, this library is not in the business of making choices of backends and passing them onto users. It is very much possibly (and anybody is very much welcomed!) to implement a PG'OCaml backend under backends/pgocaml and send in a pull request. I chose postgresql-ocaml as the first database backend implementation because it's what Inhabited Type uses internally. We could get into the reasons for that particular choice, but let's leave that for another forum.