Fixed old links to github/OCamlPro instead of github/ocaml
Added documentation on solver preferences.
Merge pull request #89 from AltGr/github-ocamlpro-links

Fixed old links to github/OCamlPro instead of github/ocaml
Merge pull request #90 from rdicosmo/doc_preferences

Added documentation on solver preferences.
Fix for tiny API change in OPAM

using functions from opened OpamTypes :/
Merge pull request #91 from AltGr/fix-api-change

Fix for tiny API change in OPAM