Import ocp-build from OCamlPro/typerex@b2a8dbddac0ae973fdf2d1f3a509d134ed84d681

This is supposed to make ocp-build work again on windows
Expose  to easily rewrite opam configuration state with a new root path after copying an opam installation
Merge pull request #603 from dsheets/master

Expose `OpamState.install_conf_ocaml_config`
make opam able to open .tar.lzma and .tar.xz archives
Merge pull request #611 from blue-prawn/master

make opam able to open .tar.lzma and .tar.xz archives
Update LICENSE to be LGPL + linking exception

Fix #612
Update CHANGES
Merge branch 'master' of github.com:OCamlPro/opam
Lazy description files.

This should improve performance

Also simplify the cache file to make it smaller (and quicker to read and write)
Add the compiler version in the metadata which are displayed on error
Improve error messages.

Display the compiler version when the current compiler is system.
Spawn a process for each update task, as it is an IO-bounded process.
Move the archive from the repo to the main folder.

This fixes an issue when dealing with HTTP repo not storing package archives. In that case, 'opam update' would mark as updated all installed packages.
Fix split update/upgrade/new/deleted messages.

Now a package can only appear in one of this categories. And we distinguish between upstream updates for packages already installed (ie. this needs upgrade) and others (simple updates).
Add failing test for 'opam repo remove'