Adding newsletters

New model `Newsletter` + new `SuscribibleDecorator`  -> I have needed to add `draper``

Also `rake suscribir:install:migrations` on dummy
You're not testing `Suscribir::NewsletterDecorator` as describe says. You're testing `Suscribir::Newsletter` as code says. So, you should move and change the test name, or change your code.