Dequeue support

Sometimes it's useful to have ability to cancel some jobs.

Now node-sidekiq supports it by `dequeue(payload)` method. For example

```javascript
var some_store = {},
    sidekiq = new Sidekiq(cli);

sidekiq.enqueue("MyJobClass", [1,2,3], {queue: 'critical'}, function(payload){
  some_store[user_session + ':some_action'] = payload;
});
```
and the cancel it using old payload:
```javascript
(function(user_session, action){
  payload = some_store[user_session + ':' + action]
  sidekiq.dequeue(payload)  // will remove the job
  /*
    also avaliable aliases for debounce:
    sidekiq.cancel
    sidekiq.unsubscribe
  */
})() // some callback

Also i've set the default namespace to `sidekiq` as a default namespace in sidekiq config.