Remove .process listeners

This is more like a question than an issue, but i'd like to know what happens with the listeners (.process in particular) when no more messages get in the queue for a given message type. For example, lets say I set a listener like:

```javascript
var kue = require('kue')
, queue = kue.createQueue();

queue.process('foo', function(job, done){
// Do something with the job
});
```
Is there a way to tell kue to stop (not temporarily pause) processing a given message type, so any memory allocated to that would be freed?

Something like:

```javascript
var kue = require('kue')
, queue = kue.createQueue();

queue.stopProcessing('foo');
```

Thanks a lot!

Start at the queue.process('foo') entry point shown in the issue and trace how the listener is registered and retained. Completion should establish whether processing for a message type can be stopped to release its resources, and document or define the behavior requested by the proposed stopProcessing('foo') API.

Written by the indexing model from the issue text.