Fixes #20: Pass should show a message

Now shows a dialog warning the opponent has passed and prompting to pass as well or continue.
This commit also includes some refactoring that was necessary for the change:
 * gameChanged now passes a Bundle with some additional info if needed (type of change or precisions such as player_passed, player_played for now)
 * a Player now has an avatar instead of just a name which make the association with the black or white stone icon cleaner and opens the way to displaying a Google Plus Avatar.