This game is about picking up packages (or passengers) and delivering
them to dropoff points on a city grid.  It fits the theme because the
Courier is connecting worlds by delivering items/messages between the
people in the city.

Implemented so far:  ability to travel on a grid, accelerate,
decelarate, and turn; ability to pickup packages (blue) and deliver
them to the dropoff points (red).  There is no timing for the
deliveries and no scoring system yet.  There is also no art or music.

Ideas for extending the gameplay:
- Timers and score more highly for faster deliveries.
- Traffic, i.e., areas of the map where you travel slower
- A braking key because the arrows aren't great for that.
- Multiple package deliveries at once, drop off the correct package
  at the correct dropoff location.