Allow to disable SwiftMailer for real

You can now completely disable SwiftMailer (which is initialized on **each** request by default) by using the new `sfNoMailer` class in your factories.yml:

    mailer:
      class: sfNoMailer

This code was sleeping on my computer - but it has been tested on some major customer websites. When dealing with emails via a service provider, RabbitMQ, or no email at all... Symfony is still creating instance of SwiftMailer. This is nuts, here is the fix for that.