Make Ember.Namespace#toString ember-cli aware

Since ember-cli [no longer assigns](https://github.com/stefanpenner/ember-cli/pull/2058) a `window.App` global, your app's `Ember.Namespace` is incorrectly registered as an empty string. i.e. `Ember.Namespace.NAMESPACES_BY_ID['']`. To fix this and allow for easier debugging, this makes it so ember is ember-cli "aware", using the `modulePrefix` property if available.

You can then get a reference to your app via `Ember.Namespace.NAMESPACES_BY_ID['app']` or whatever your name is.

*Alternatively, ember-cli could stop using `modulePrefix` and use `name` instead, and define _that_ in the `app.js`. IMO that's better to me, but figured there was some legit reason for using `modulePrefix` instead?