Now performing tagged logging on invoke_job.

@tucker250 

[`Delayed::Backend::Base#name`](https://github.com/collectiveidea/delayed_job/blob/master/lib/delayed/backend/base.rb#L75) seemed like a good candidate for the logging context.

Also, you'll notice that I'm using `Rails.logger`. This is because the `Delayed::Backend::Base` module cannot access `Delayed::Worker#logger`.

Let me know if you've got any questions.