Add the ability to view and interact with the crawler queue

[bhermann]

Crawlers will produce and work on items in work queues.
In the management application there should be a way to view these queues, to pause them, to rearrange items, and to delete items.

[bhermann]

I am almost certain now that we should use Akka Actors to implement this. This means I will rewrite the issues with the word 'queue' in it.