Start webbrowser in a thread.  Prevents lockup with Chrome.
Start webbrowser in a thread.  Prevents lockup with Chrome.

If a user has Chrome set as their default browser (system-wide or via
the `BROWSER` environment variable), opening the notebook hangs
because the chrome call doesn't return immediately.  This solves the
issue by opening the browser in a thread.

Note that there remains an issue where killing the notebook will kill
Chrome if the Chrome session was started by us.  I haven't found a way
to work around that despite attempts by making the webbrowser.open()
call in a subprocess.

ipython/ipython,ipython/ipython