Updated delay extension for ActiveRecord instances

In reference to #278.

Given the recommendation to not queue up instances, this pull request instead queues up the class name, primary key, method name, and args for the ActiveRecord delayed extension. Processing the job then does a fresh `find` and sends the along the method/args.

This is more similar to the `delayed_job` approach - allowing for the elegance of `model.delay.method` without the worry of queuing up the actual instance.