API change: OpamFile.Urls_txt -> OpamFile.File_attributes

This module can be used in other settings (opam update for instance, to store local checksums in a cache)
Add regression tests for #676
Add a way to install lib, doc and such in sub-dirs.

Fix #676
Update the dev manual
Update man pages
Update API docs
Support for `$prefix/lib/stublibs` in `.install` files
Update the dev manual
Update the doc API
Update CHANGES
Merge branch 'master' of git://github.com/OCamlPro/opam
Update the doc regarding #673 fix
Add a `man` field in `.install` files

Fix #673

The default behavior is to copy the files in `/man/man3/$file`, you can overwrite it by adding the optional argument.
Update API doc after the fix of #673
Update CHANGES
Merge branch 'master' of git://github.com/OCamlPro/opam
Fix typo
Never set OPAMSWITCH in ~/.opam/opam-init/variables.*

Fix #677
Accept OPAM_foo=1 to tell OPAM that `foo` is true or OPAM_foo=0 that is false.

(OPAM_foo=true and OPAM_foo=false are still valid)
Add new package variables.

Fix #674

I've added:

    opam config var core:version
    opam config var core:pinned
    opam config var core:bin
    opam config var core:lib
    opam config var core:doc
    opam config var core:share

and it's easy to add new ones