Change `EventEmitter` to `Stream` for ".pipe()" and other Stream goodies

As explained in [here](https://github.com/aredridel/node-smtp/pull/5) it becomes handy to have a Stream inherited message. The pipe method can really come handy in much ways.

For example I used it in combination with [mailparser](https://github.com/andris9/mailparser) module like this:

```javascript
//...
imap.fetch(results, {
  headers: {
    parse: false
  },
  body: true,
  cb: function(fetch) {
    fetch.on("message", function(message) {
      var parser = new mailparser.MailParser;
      
      parser.on("headers", function(headers) {
        return console.log("Message Headers:", headers.received);
      });
      
      parser.on("end", function(mail) {
        return console.log("Mail:", mail);
      });
      
      message.pipe(parser);
    });
  }
}, function(err) {
  imap.logout();
});

//...
```