Adding newsletters

New model `Newsletter` + new `SuscribibleDecorator`  -> I have needed to add `draper``

Also `rake suscribir:install:migrations` on dummy
roger! I prefered extending instead of inheriting, but the idea of making clear that it's not a Decorator has been done: https://github.com/Soluciones/suscribir/commit/59a7b1bbd7f5ff99e7f7a585a513d8ebf0d05a67