use Promises for tasks

We're writing majority of code using async/await these days. So our first thought was to use it for kue-tasks too. Something like this:

``` js
queue.process('hello', async (job) => {
let tmpResult = await someLongBoringOperation(job.data.name);
await otherLongOperation(tmpResult, job.data.planet);
});
```

That doesn't work as kue doesn't distinguish promises from regular functions. Workarounds are mentioned in Readme, but they hurt elegance of app code.

Is there a chance that kue will have "native" support for promises?

The only complication I see is optional `workerCtx` param. Otherwise, the code should be trivial

Start with the README's documented workarounds and the queue.process entry point shown in the issue. Trace how task callbacks and the optional workerCtx parameter are handled, then define completion as native support for async functions without the current application-level workaround.

Written by the indexing model from the issue text.