Update configure files to substitute the man-pages
Update the generated ./configure
Fix #561

(was missing the -s option, to generate stand-alone man pages)
Update the generated man pages
Update the tutorials
Add a link to the online documentation in README.md
Improve the documentation for OpamHeuristic
Update HTML doc for OpamHeuristic
Improve the documention of OpamHeuristic (cont)
Fix typos in previous commits, rework some sentences.
Update API doc
Update doc
Display the full SHA1 instead of a random string when using `opam --git-version`
Ignore generated files.
Merge pull request #581 from Chris00/master

Ignore generated files.
Merge branch 'master' of github.com:OCamlPro/opam
Fix the solver behavior in case of choices with conflict.

Moreover, when we a package is an implicit dependency, we try to keep as much as possible the version already installed.

This is done is 4 steps:

1. we explore the state space only for the package explitely given on the command-line
2. we iterate on the installed packages which are in the dependency cone, to check if they are still installable
3. we order the remaining packages in the dependency cone by version number (to favor newest packages instead of deprecate ones) and we incrementally explore their state space to check (i) if the package is really necessary and (ii) find the best version available
4. we incrementally iterate the already installed packages (not in the dependency cone) to check whether they need an upgrade (or a downgrade)

This is to work well on my limited testing (eg. after playing a bit with old versions of core, cow and oasis2opam)

Fix #480
Update debug statements
[Client] packages can specify messages to print during summary

Add a messages field that allows to describe which feature is missing
when an optional dependency is not installed (issue #572)

For instance:

    messages: [
      "The optional realizations and why3's coq plugin will not be available since coq is not enabled" { !coq:installed }
      "The optional selection hypothesis plugin will not be available since ocamlgraph is not enabled" { !ocamlgraph:installed }
    ]

Reviewed by @samoht
Improve the syntax of filters.

Now is should be similar to the build field, where "%{X}%" can be shortened to X. Also, allow `!`

This is useful for #572