Add optional client_id to the identify

Adds an optional `client_id` to the "identity" by exposing a new `id` option to readers ([see spec](http://nsq.io/clients/tcp_protocol_spec.html#identify)).

```js
var reader = nsq.reader({ id: 'my-worker' });
```

Nice little benefit of this is that you can get a littttttle better idea of who is who in the admin, when multiple consumers are running on a single host. See before and after:

__Before__

![no-client-id](https://cloud.githubusercontent.com/assets/67717/3978775/61000904-284c-11e4-9d33-915959c10bae.jpg)

__After__

![client-id](https://cloud.githubusercontent.com/assets/67717/3978777/67e84a38-284c-11e4-8931-790458503c26.jpg)

cc: @visionmedia