Make ActionMailer delivery_method plugin-able

currently we have the following syntax

config.action_mailer.delivery_method = :smtp
config.action_mailer.smtp_settings   = {
  :domain => 'somedomain',
  :address            => "mail",
  :port               => 25,
  :perform_deliveries => true
}

but I propose we should be able to do the following

config.action_mailer.delivery_method = MyCustomerDeliveryMethod.new

attached is a patch that makes ActionMailer::DeliveryMethod which is a namespace for a collection of implementations
if done right it will be easy to deprecate all these different "smtp_settings", and "file_settings" and such
and replace it with the following consistent syntax

config.action_mailer.delivery_method = :smtp, {:some => "options"}

and

MyMailerClass.delivery_method.settings = {:some => "overrided options"}

the second half is not included in the patch,
but would be simple for me to implement,
if you thought it was a good idea.