Adds an 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.
I don't generally think that we need a local function for this.  I believe that the project should be initialized in the `run` function, and just be asked for its path's before `lookup` or `list`.