Label promises

Add labels to promises.   This PR is part of https://github.com/tildeio/ember-extension/pull/76

To try out the labels, clone the Ember inspector [promise branch](https://github.com/tildeio/ember-extension/tree/promise) and visit [this app](https://s3-eu-west-1.amazonaws.com/teddy-share/promise-app/index.html#/ember_data) .