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**.

Closes #75.
Updated to use `ember-cli-addon-main` (will need to wait for https://github.com/stefanpenner/ember-cli/pull/1052 to be merged).