Merge pull request #850 from samoht/master

Improve the heuristic to handle cases where wish_remove <> [] AND wish_u...
Better error message when errors encountered in 'installed' files
Fixed yet some cases of upgrade with removed packages versions
The 'reinstall' can now process normally using the caches, instead of
removing packages like 'upgrade'
Merge pull request #851 from AltGr/yet-more-removed-pkgs

Yet more removed pkgs
Merge pull request #853 from samoht/master

The release is almost there ...
Merge branch 'master' of github.com:OCamlPro/opam
Update docs