Improve addon support

Since a lot of the Ember testing ecosystem relies on this addon, seems like there could be a bit better support behind it. In particular I feel like the following could be beneficial:
- [ ] Document public/private APIs (everything appears public currently)
- [ ] Add tests
- [x] ES6ify the code (it is using ES6 modules but everything else is ES3 or so)

If these seem desirable I can work on implementing them. Though I'd need a bit of guidance on the documentation front.