Better orphans computation

Previous version was too expensive (calling the solver), and too strict
(duplicated checks done at other places).

This should be faster and easier to understand.
cleanup OpamClient.update: don't pass string as OpamRepositoryName.t

wait until we can decide to convert the argument to repo or package name
More robust handling of pinning commands

Handles re-installation and re-pinning with metadata changes
Interactive handling of pinned package metadata updates
Improved stats display
Merge pull request #1298 from OCamlPro/fix-orphans-again

Fix performance and simplify orphans computation
Merge pull request #1300 from OCamlPro/cleanup-overlays

Sanitize pinned package handling
Fixed '??' versions appearing in conflicts messages
Enable backtraces in travis