Rework file & module structure to be built around and within the PostmarkRails module.

This avoids polluting the global namespace with un-namespaced modules.
Use ActiveSupport::Autoload so that we don't even load the PostmarkRails::DeliveryMethod module in newer versions.
Since the readme itself says we don't support rails < 3, drop the old PostmarkRails::DeliveryMethod module.
If rails is defined, use a railtie, so initialization will be tidy.