Add `files` to package.json / Update mocha

* I added `node_modules` directory to `.gitignore`
* I added [`files`](https://www.npmjs.org/doc/files/package.json.html#files) field to package.json. `test.js` and `.travis.yml` etc. don't have to be included in the npm package.
* I updated [mocha](https://github.com/mochajs/mocha) to v2.0.1.