Support a client in node.js

Currently if I try to use this library as a client in node.js, like this:
```
var websocket = require('websocket-stream')
websocket('ws://localhost:)
```

I get:
```
ReferenceError: WebSocket is not defined
      at new WebsocketStream (/Users/matteo/Repositories/MQTT.js/node_modules/websocket-stream/index.js:21:19)
```