Add recording of job queued at times

For monitoring Sidekiq i'd like to measure how long the oldest job has been waiting in the queue. I think this is a better measure than the amount of jobs waiting. 

With this patch I can do something like this to work it out:

```ruby
def max_sidekiq_wait_seconds
  queues     = Sidekiq::Stats.new.queues
  wait_times = queues.map do |queue_name, job_count|
    if job_count > 0
      jobs = Sidekiq::Queue.new(queue_name)
      (Time.now - Time.parse(jobs.first.enqueued_at)).to_i
    end
  end

  wait_times.max || 0
end
```

I've added a test for the API class but let me know if you'd like some more unit testing.