Tweaked some state-related settings.
Implemented difficulty settings with logarithmic scale for increase in difficulty.
Changed internal difficulty setting to be represented by a number, rather than a string.
Added information to the development info panel (orbiting ships, live missiles, and total missiles fired).
Changed scoring method so more points are given for kills at more difficult levels.
Changed high scores list so that the difficulty level is indicated for each high score entry, as well as the current score.
Randomly invert ship order/direction at start of new games.