Add queue attribute run_delay

[johnnyshields]

• Add queue attribute run_delay which is a default time offset/delay by which to run the job
• light refactoring to JobPreparer
• additional specs covering queue attributes from JobPreparer

The use case for this is that I'm using MongoDB and due to how replica set work (non-transnational) there are occasional mailer job failures when objects havent been replicated across all replica sets.

[johnnyshields]

The spec failure here is not related to this PR, please check travis.

[albus522]

This is an interesting idea, but I think you can get the result you want using DJ lifecycle callbacks which are unfortunately not well documented.

Adding something like this in your DJ initializer should do what you want

Delayed::Worker.lifecycle.before(:enqueue) do |job|
  if job.queue == "my queue"
    job.run_at ||= 10.seconds.from_now
  end
end

This also allows you to use other attributes of the job than just the queue name to determine how much you want to delay things.