Update index.html.

Add missing favicon, trim spaces.
Losslessly compress close.png.

1156 bytes --> 280 bytes.
Add an .npmignore file.

This prevents installing files not needed by the end users.
Add a package.json.

Now you can do `npm install` and `npm run build` to run coffeescript.
Add JSHint support.

Use `npm run check` or `npm run lint` to check the js/lightbox.js file with JSHint.