Added streaming to ImapMessage.

[daxxog]

I added easy access to my imap-message-stream class.

Pipe example

//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);
});

[mscdex]

(streams2) Streams are now passed since the rewrite.