OCamlPro logo: cropped and made transparent
Add a few OCamlPro logos, and update commercial offer description
OCamlPro: reviewed the changes as per PR comments
companies.md: list Facebook, Bloomberg, Citrix on top

Addresses #345.
Merge pull request #316 from AltGr/ocamlpro-visibility

Following our discussion, improving OCamlPro's visibility a little bit.
Merge branch 'master' of https://github.com/ocaml/ocaml.org