libspotify: add darwin library
install_name_tool: add expression
pyspotify: fix build on darwin

* use install_name_tool to have _spotify.so point to the correct library
pyspotify: update to latest version
Merge branch 'libspotify' of git://github.com/lovek323/nixpkgs into lovek323-libspotify

Conflicts:
	pkgs/top-level/all-packages.nix