Allow enqueue_job to override the synchronous setting on a queue. Add option to the @job decorator.

Makes enqueueing a single job synchronously easier than creating a second queue object, and allows for the ```@job``` decorator to use a setting for asynchronicity. This only allows overriding in the sync > async direction.

Ex: 

```

from settings import JOB_ASYNC

@job('low', async=JOB_ASYNC)
def do_something(...):
    ....
```

_(Edited for example code formatting)_