Upgrade opam to 1.1.0

Changes since 1.0
-----------------

* `opam config env` now detects the current shell and outputs a sensible default if no override is provided.
* Improve `opam pin` stability and start display information about dev revisions
* Add a new `man` field in `.install` files
* Support hierarchical installation in `.install` files
* Add a new `stublibs` field in `.install` files
* OPAM works even when the current directory has been deleted
* speed-up invocation of `opam config var VARIABLE` when variable is simple (eg. `prefix`, `lib`, ...)
* `opam list` now display only the installed packages. Use `opam list -a` to get the previous behavior.
* Inverse the depext tag selection (useful for `ocamlot`)
* Add a `--sexp` option to `opam config env` to load the configuration under emacs
* Purge `~/.opam/log` on each invocation of OPAM
* System compiler with versions such as `version+patches` are now handled as if this was simply `version`
* New `OpamVCS` functor to generate OPAM backends
* More efficient `opam update`
* Swtich license to LGPL with linkink exception
* `opam search` now also searches through the tags
* minor API chanages for `API.list` and `API.SWITCH.list`
* Improve the syntax of filters
* Add a `messages` field
* Add a `--jobs` command line option and add `%{jobs}%` to be used in OPAM files
* Various improvments in the solver heuristics
* By default, turn-on checking of certificates for downloaded dependency archives: use `./configure --disable-certificate-check` to go ba
ck to the previous behavior
* Check the md5sum of downloaded archives when compiling OPAM
* Improved `opam info` command (more information, non-zero error code when no patterns match)
* Display OS and OPAM version on internal errors to ease error reporting
* Fix `opam reinstall` when reinstalling a package wich is a dependency of installed packages (regression introduced in 0.9.5)
* Export and read `OPAMSWITCH` to be able to call OPAM in different switches
* `opam-client` can now be used in a toplevel
* `-n` now means `--no-setup` and not `--no-checksums` anymore
* Fix stupid typo in detection of FreeBSD archs
* Fix installation of local compilers with local paths endings with `.../ocaml/`
* Fix the contents of `~/.opam/opam-init/variable.sh` after a switch