build#247 - Pick Your Difficulty

Game now has difficulty levels.
Harder difficulty means faster movement, more damage taken from enemies/obstacles, and more points earned (each difficulty-level has its own score-multiplier).

Also, lots of small (but annoying) gameplay bugs fixed.
Most noticeable one was inability to shoot through dead/dying enemies -- your bullets were still colliding with (already dead or dying) things.
And fuel explosion were hitting only first thing it reached, which was wrong and not fun. Now, shooting fuel pickups (and causing huge explosion) has become a viable tactic for certain situations.