Merge pull request #372 from tuong/master

Fix the non-termination of OPAM when sometimes we install an arbitrary package, see 006293e1f8bfb317dab59308cabe4363c6b67675 .
Improvement to the solver heuristics:

* Reduce the size of the state space explored by the brute-force heuristics.
* Minimize the actions perfomed by OPAM to only the one explicitely required by the user.

This should fix #323 (or at least greatly improve similar situations).
Merge branch 'master' of github.com:OCamlPro/opam
Update CHANGES
we continue 7fcfe9c5ae746fe694f9fcab4d3f0fff3db84f42