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.
Just noticed that users currently can't select which PHP version to install. Our current version will just install the latest version. This PR is not the place to fix this, though, so I opened #25.