Refactor: Enable EmberGenerator to pass app_name to EmberBuilder

This change is so that the app_name is available as an attribute
in EmberBuilder, and app_name does not have to be injected into
each and every action method.

An accessor is added to EmberBuiler and app_name is injected into
an EmberBuilder instance creation call, e.g.

`builder = EmberBuilder.new( app_name: app_name )`