Fixes broken collision detections
Adds weapon pickups.

Weapons have their own position and draw method (TODO: inherit this
from something else--possibly MovingObject, though it doesn't move--
maybe a yet-undetermined prototype of them both).

Game also has a pickups array. When it detects that the ship collided
with a pickup, it calls pickup.pickedUpBy(ship). Weapon implements
that method.
Weapon doesn't need to know about Game anymore

Game removes the pickup in its collision detection loop, rather than
making the pickup itself call game's removePickup method.