Inline worker and Sidekiq worker documentation.

Introducing a Sidekiq worker without depending on Sidekiq is too buggy to be worth maintaining (conditional autoloads and checking for Sidekiq in the environment `===` bad).

As a nice side effect of only offering an inline worker as the default, all the `process_async` vs `process_now` code goes away, making things easier to test.

Instead, I added a base worker, a mechanism to override the default worker from the controller, and an example Sidekiq worker in the README.

@srbiv :eyes: