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.
Yes, I agree. I got "inspiration" from #1196 to fix the tests this way.
I can have a look at the tests tomorrow night. Or maybe @jakecraige can do it earlier?