title: Socket.IO subscribe to multiple channels
body: <p>I want to build a simple chat room system on top of Socket.IO where user can create a new chat room and then people can start chatting.</p>
  
  <p>This sound simple but as the Socket.IO 0.9.4 I'm running now, after reading a few SO posts together with the document on socket.io, i'm getting more and more confused. So, I hope that someone can provide me with instruction that WORK with 0.9.4:</p>
  
  <ol>
  <li><p>I need a way to subscribe to a room. The room name is chosen by user. When a message is posted in a room, the user should receive it. How should I write the server code, how should I write the client code?</p></li>
  <li><p>A user can join multiple rooms simultaneously.</p></li>
  <li><p>I want another system to send a message to all user in a certain room. This 'another system' send the message through a request handled by express. How would I write that request handler?</p></li>
  </ol>