Addon API for blueprint packages.

Adds an addon api for blueprint packages.
    
A blueprint addon must:
* specify the keyword `ember-cli-blueprint` in `package.json`
* contain a blueprints folder

Everything else is automagical. Just npm install the blueprint package and use it's blueprints.
And it looks like that was the main change here, so perhaps we should roll this func back to the way it was, and fix the tests (which is I assume why it was added).