Allow Webpack support

When requiring cheerio in a webpack bundle, two issues arise:

 - Firstly, the `entities` module loads JSON files, which trips up webpack. This is easily circumvented by using [json-loader](http://npmjs.org/package/json-loader).
 - Secondly, in `index.js` where `package.json` is required. The lack of explicit file extension here prevents webpack from detecting that this is a JSON file, not a JS module; without this it cannot discern the correct loader to use.