Upload #4 with many improvements

So many improvements.

• Added seven new buildings to the cities - there's much more variety, and covers all the building types from my original notes.
• Added a van and a pickup truck. Vans take 5 hits, pickup trucks take 4.
• When you bump into a hospital (white building with a red cross) or a garage (gray building with the blue car on it), you heal.
• The AI's navigation is much faster
• The AI checks to see if the line of fire to the player is blocked by a building before firing
• Fixed a bug with the health display being drawn overfull when at full health
• - and = keys for volume control