Nix package manager; Improve install.sh.

Just as a backup, use Nix instead of Homebrew/Linuxbrew. In some linux
boxes I have problems with linuxbrew.

It doesn't have all packages, but it's a start.

http://blog.errright.com/2014/07/27/switching-from-homebrew-to-nix/
https://lists.debian.org/debian-devel/2008/12/msg01027.html
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