Fixed up being able to pass random headers in with headers, or mail.  Also, undeprecated headers(hash) as this works now too

Added delivery_handler method to mail and implemented in ActionMailer to deliver inside of instrumentation

Add active_model/railtie back to generated boot.rb, add models back to paths, load active_support/railtie since we need it and ensure default logger is set before config.

Adding failing tests for calling just the action, instead of :create_action_name and :deliver_action_name

Delegated ActionMailer::Base.deliveries to Mail.deliveries, added callback support in Mail to call ActionMailer on delivery, moved deliver to deprecated API in preparation for new API