Improve addon support

[trentmwillis]

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
• 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.

[rwjblue]

Totally agreed with all points!!!

[rwjblue]

(Optional) ES6ify the code (it is using ES6 modules but everything else is ES3 or so)

Done in #34