abstract all of the ActionMailer delivery methods into their own classes. thereby the following are equivalent

  ActionMailer::Base.delivery_method = :smtp
  ActionMailer::Base.delivery_method = ActionMailer::DeliveryMethod::Smtp

we could equally set our own custom object
as long as it provides the instance method :perform_delivery(mail)

eg.

  class MySmsDeliveryMethod
    def perform_delivery(mail)
      Sms.send(mail['to'], mail['body'])
    end
  end

  MySmsMailer.delivery_method = MySmsDeliveryMethod.new
Signed-off-by: NJosé Valim <jose.valim@gmail.com>