No mention of Tornado requirement in README

It was a simple fix but initially I got this:

```
~/iOS-HTML5-Tethering:master> sudo python client/client-ws-server.py
Password:
Traceback (most recent call last):
  File "client/client-ws-server.py", line 3, in <module>
    from tornado.websocket import WebSocketHandler
ImportError: No module named tornado.websocket
```
