Remove CoffeeScript

[adam-lynch]

We're going to use plain JavaScript in this repository from now on. Why?

• It makes it easier for people to contribute.
• No build step needed.
• Forks are easier to test because the JavaScript is in the repository.

So the existing CoffeeScript needs to be converted to JavaScript. Nothing too fancy, we don't want to have to introduce babel or require users to have Node 8 as a minimum or anything like that.

In doing this, gulp could be removed too but the tests need to still be runnable via npm test.