Addon command and blueprint template

This pull request adds an addon command to ember cli and a blueprint for generating a default addon directory structure. The addon blueprint is setup to use the new addon API so that it can easily be used by a consuming application.

The addon command works like how you would generate a Rails engine: `ember addon new addon-name`.

The addon blueprint uses a similar app structure as Rails engines uses for testing. There is a tests/dummy/app for doing acceptance testing of the addon to mimic how it would be used in a consuming application. 

The setup is based heavily on the one used for `ember-cli-super-number`, other than I used a more Rails like directory structure based around how Rails engines are structured. Also by using the `app` directory ember-cli generators work as expected without having to make any changes.