stream output from celery

With this commit we begin using girder jobs within romanesco.  We store stdout/stderr output from celery into girder, and can stream it back out to the web via a new REST endpoint.

Two questions for you @zachmullen:
1) As you can see, I wholesale lifted your StdoutLogger class into romanesco.  Is there a more appropriate place where this should live?

2) You'll also notice in the function ``romanescoRunOutput`` that I had to copy a substantial chunk of your Notification API model to enable streaming of stdout to the web browser.  Any preference on what approach we should take to eliminate this code duplication?