Hack CSS to improve readability on a mobile device.

* example/index.html: Add viewport information to set width to
  device-width and initial-scale to 1.
* res/default.css: If width is below 480px:
  - hide roster-pane and toolbar,
  - use full width for message-pane and message-form,
  - put the number of users icon on top right,
  - clicking on this icon make the roster-pane appear.