A streaming API of job console output

Hi,

I'm about to implement a simple continuous deployment dashboard integrating with Jenkins and I just stumbled upon nestor. A readable stream would be easier for me to implement a stream -> websocket proxy for browser. Luckily the current implementation is fairly complete and all I have to do is a little refactoring to get this far.

However, this pull request is by no means complete:

* ConsoleStream doesn't fully implement the readable stream API yet. But currently it works well enough for me.
* Not sure how to write tests for this :( But I did test it manually for a bit.

Feel free to tell me what more should I do to get this merged.