Add callback for enqueue()

Hi,

I found it's useful to add callback for enqueue() function because we're able to know whether tasks are enqueued successfully and how many remaining tasks in the queue. Please consider the following code:

```javascript
resque.enqueue('math', 'add', [1,2], function(err, remainingJobs) {
    console.log('New job queued. Remaining jobs in queue: ' + remainingJobs);
});
```

Thanks for your awesome lib!