Added streaming to ImapMessage.

I added easy access to my [imap-message-stream](https://github.com/daxxog/imap-message-stream) class.

Pipe example
-------------
```javascript
//see https://github.com/mscdex/node-imap#example

fetch.on('message', function(msg) {
    fileStream = fs.createWriteStream('msg-' + msg.seqno + '-body.txt');
    msg.stream.pipe(fileStream);
});
```