Drop rails 2 support, and use a Railtie so that initialization will be appropriately timed.

I haven't tested this in production myself, but it passes the specs and is worthy of discussion.

This was prompted by me running into some trouble with postmark failing to send after moving my configuration from application.rb into an initializer. It shouldn't have mattered but it did, and I think the culprit was the non-standard rails initialization you guys use.

Let me know what you think.