Publish as an npm package

This patch adds a `package.json` file and exposes `fuck.js` as the binary. This means three things:

1. you can now [publish](https://npmjs.org/doc/publish.html) this package on npm
2. people will then be able to `npm install jsfuck` and `require('jsfuck')` in their code
3. people will be able to install jsfuck globally (`npm install jsfuck -g`) which allows them to use the `jsfuck` command in their shell