Prevent duplicate jobs

To avoid triggering more work than is needed, I'd like to be able to prevent dup jobs.
I've heard suggestions in other issues to record and check existing Job Ids, and then not create the job, but this isn't practical.

Conceptually, it seems like the solution is to let consumers provide a Job ID when enqueueing a job:

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

var dedupedJob = queue.create('trimList', { arg: 'xxx' }, { id: 'myUniqIdentifier_whichWillPreventDupsUntilProcessed' });
dedupedJob.save();
```

See: #1034

Start at the queue.create enqueue path shown in the issue and review the related discussion in #1034. Determine how a consumer-supplied Job ID should prevent duplicate creation until the existing job is processed; done means the proposed API behavior is implemented and covered by appropriate queue tests.

Written by the indexing model from the issue text.