Class level logging

Based on the [error handling article](https://github.com/mperham/sidekiq/wiki/Error-Handling) in the Sidekiq wiki. I should be able to use the Sidekiq logger in class methods such as `sidekiq_retries_exhausted`. I found this not to be true earlier today. Definitely open to feedback on the implementation. I didn't want to just copy the logger method into the `ClassMethods` module.