EmberAddon

Will call the EmberApp constructor with defaults for addon projects. In
the addons Brocfile only the following is required:

```js
var EmberAddon = require('ember-cli/lib/broccoli/ember-addon');
var addon = new EmberAddon();
module.exports = addon.toTree();
```
Merge pull request #1343 from bcardarella/bc-ember-app-addon-constructor

EmberAddon