RFC: Gentoo support

This is a Request for Comments for Gentoo support, the code below is not quiet ready and still needs some more testing.

Some things like the multi-version support may be useful for other distros as well while the extension to USE flag mapping is totally Gentoo specific.
A better way for handling the package installation may be a virtual resource, but I had to touch your code in too many places for that.

But maybe someone has additional ideas/suggestions.