Added an emitter option to the Client

Allows you to inject an emitter to the client, so you can pre-attach listeners and subscribers.

```php
$emitter = new Emitter();
$emitter->on('before', function($e) {die('BAIL!');});
$client = new Client(['emitter' => $emitter]);
```

This solves the root issue of #639 in a backwards-compatible way.