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