Improve messages on update
Improve the detection of cyclic dependencies.

This should improve 932e6b81d98e8838784d471712fad6114f7566d1
Better error messages when something goes wrong in OpamFilename
copy_dir actually does something different than its name could let think of: rename into copy_unique_dir and add a real copy_dir function
Fix #440
Update CHANGES
Fix #435

For pinned packages, 'opam reinstall <pkg>` is now a shortcut to 'opam upgrade <pkg> && opam update <pkg>`.
Fix unit-tests.

The cache handling is quite sensitive to change, need to put some big flashing warning around this I guess ...