Improved install.fr.md page

* Fix latest version
* Translate OPAM usage
Merge pull request #407 from lefessan/install-french

Improved install.fr.md page
Improve French version of mailing_lists.md
The first page in French
Add alt tags on images
Improve index.fr.md baseline
Merge Lefessan's contributions to the French Mailing List page
Add translation "buttons" to the front (core) page
Merge branch 'master' into index.fr.md
Fix to use standard translation links
Small improvements to the French front page
Merge remote-tracking branch 'ocaml/master' into translate-navbar
Use a translation file

Translations will be stored in translations/en_to_??.txt with the
syntax:
"english" -> "french" (* or other language, with OCaml comments *)