Tell Node.js which file to load

This enables Node.js to load the file when installed through NPM as Node.js needs to either be told through the package.json file which file to load for a module or there needs to be an index.js file in the folder – otherwise Node.js doesn't know which file it is that's the entry point for the module. Related documentation: http://nodejs.org/api/modules.html#modules_loading_from_node_modules_folders