Make it possible to create javascript widgets from the sage cell

* updated to most recent version of ipython comm pull request
* make a new javascript function that will execute javascript in the browser
* make a working comm example (put in todo.rst file)
* fix showstopper bug in send_message (inappropriate this)
* enable console.log in compute_server code.  IPython still prints out stuff to the log.  We'll need to submit a pull request to stop IPython from being so chatty.