Allow addons to add blueprints.
Allow blueprints to export an object.

This allows blueprints from addons to have access to all of the internal
inherited functions from the EmberCLI `Blueprint` class without having
to have `ember-cli` as a dependency.

Also, make `addon.blueprintPaths` a function.
Merge pull request #1222 from rjackson/blueprints

Allow Addons to provide blueprints.