Rename ember cpm to index

Renamed addon/ember-cpm.js to addon/index.js.

This allows to import all the ember-cpm namespace using
```js
import EmberCPM from "ember-cpm";
```
instead of
```js
import EmberCPM from "ember-cpm/ember-cpm";
```

This is a breaking change, but is the way it should be done. I should have read about ES6 spec and how transpilers work earlier.
Hopefully is the last breaking change for a looong time.

Also, fixes a bug with the macros bluprint.