Add:
    - bulletMove() function, which takes a bullet and moves it according
    to direction of motion.  It removes the bullet if it is touching a
    wall
    - enemyMove() function, which takes an enemy and moves it according
    to the direction of motion.  It changes the direction of the enemy
    if it runs into a wall.
    - all new functions to the header file
Mod:
    - all surface collision functions
    - they now take a Geometry object, so they can be applied to enemies
    and bullets as well as players