Documentation

Reading the doc, I have several points of uncertainty :
* what is a "unique queue" ?
* what do you mean by the "atomicity" feature ? A failed task is not removed from the queue ?
* what do you mean by "persistent queue" ? Isn't this redis-based ? (so it depends on redis config for persistence)
* what do you mean by "unique tasks" ? How a task could be not unique ?
* what is the prototype of the `done` callback ? implied node-style ?
* what exactly does queue.close() ? Does it discard non-processed jobs ?

It would be nice to have those points covered in the doc.