Reimplement Sidekiq worker

This better integrates various Sidekiq features into AJ jobs.

Things like the JID will be set as expected and the user can use `sidekiq_options` in AJ::Base subclasses as usual to configure various features.
Remove all Sidekiq-specific stuff from job, enable retries by default
Merge pull request #38 from mperham/rework_sidekiq

Reimplement Sidekiq adapter