Require js

I wasn't sure of the correct licence so you might want to update that inside package.json.

Also I noticed there is now an error in the console when running the tests `Uncaught ReferenceError: module is not defined` due to having added this: `module.exports = Liquid;`. I'm not sure of the best way of addressing that.

Oh also if you run `npm test` it will open the test suit in a browser provided your system accepts the `open` command.