Use defeatureify to strip debug messages.

Recent changes to `defeatureify` allow stripping of debug messages. This
will allow us to use multi-line debug messages (for example).

To enable this the format of `features.json` has changed.

Before:

```javascript
{
  "foo-feature": true
}
```

After:

```javascript
{
  "features": {
    "foo-feature": true
  },
  "debugStatements": ["Ember.warn", "Ember.assert", "Ember.deprecate", "Ember.debug", "Ember.Logger.info"]
}
```
Merge pull request #118 from rjackson/use_defeatureify_to_strip_debug

Use defeatureify to strip debug messages.