Update to serve as Ember CLI Addon.

* Add Ember CLI addon.
* Add grunt copy task to prepare vendor/ subdirectory for publishing to
  NPM.
* Adds `ember-cli-addon` keyword (and `firebase`).

You will need to publish to NPM, but then once 0.0.35 is published users
can simply run `npm install --save-dev emberfire`, and it **just
works**.