nix-prefetch-scripts: new expression

The various nix-prefetch-* scripts are quite useful to obtain hashes for the sources of 
packages, and I don't want to search them inside nixpkgs each time I need them, so I 
packaged them. `nix-prefetch-url` is already installed with `nix` itself, so that's not 
included here.
No dots at the end of descriptions please.