added sloc

I packed the node cli app sloc [sloc](https://github.com/flosse/sloc) with npm2nix and `nodePackages.buildNodePackage`.  However there is an alternative way to do, e.g. like @matejc [tried it](https://gist.github.com/matejc/54a793979455c265baaa). Which way do you recommend for the future?
then it would make sense to change this line:
```
buildInputs = [ pkgs.nodePackages.coffee-script ];
```
to this line:
```
buildInputs = [ nodePackages.coffee-script ];
```

AND

this line:
```
${nodePackages.coffee-script}/bin/coffee -o $out/lib/node_modules/sloc/lib/ -c $src/src/
```
to this line:
```
coffee -o $out/lib/node_modules/sloc/lib/ -c $src/src/
```

by adding package to `buildInputs` it is already added to PATH variable, doesnt make sence to use one package and specify another since `pkgs.nodePackages.coffee-script` is not the same as `nodePackages.coffee-script`