Use an addon to bring in ic-ajax.

Uses https://github.com/rjackson/ember-cli-ic-ajax to provide ic-ajax as an addon.

There are at least a couple of benefits here:

* Adding/removing ic-ajax is now a one step operation (`npm install --save ember-cli-ic-ajax`).
* We have a logical place to put custom logic (in the future) to ensure `$.ajax` is not used (as it is not test friendly).