libspotify: add darwin library

I've also fixed `pyspotify` for darwin in this PR

This won't build successfully until https://github.com/NixOS/nixpkgs/pull/720 is merged.

* `install_name_tool`: add expression
* `libspotify`: add darwin library, add install_name_tool expression
* `pyspotify`: use install_name_tool to have _spotify.so point to the correct
  libspotify.dylib, update to latest version