Tweaked in a few Node.js conventions

Thanks for uploading your awsome sprintf package to NPM, I love `sprintf()`!

I took the liberty of adding `node_modules/` to `.gitignore` so that git doesn't try to version control sprintf's dependencies.

I also configured `package.json` for nodeunit, so all you have to do to test node-sprintf is download the source and run `npm install && npm test`.