Use project.root for ember generate

Addresses #1153

Refactors Blueprint. Here are the details:

Refactor Blueprint Lookup:

The only default lookup path is ember-cli’s own blueprints directory.
Any other lookup paths must be passed explicitly.

    Blueprint.lookup('foo', { paths: [project.blueprintsPath()] });

Refactor Blueprint Construction:

The constructor takes only the path to the blueprint directory. The
options `ui`, `analytics` and `project` should now be passed to
`install`.

    var blueprint = new Blueprint('my/cool/blueprint');

    blueprint.install({
      target: this.project.root,
      entity: { name: 'foo' },
      ui: this.ui,
      analytics: this.analytics,
      project: this.project
    });