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.
This function name is just slightly different from `blueprintsPath` (which is the path for the projects local blueprints). Can you change the `blueprintsPath` function to `localBlueprintsPath`, and change this to `blueprintPaths`?