Updates TODO
Adds Timer object, used in Game

Timer is an object that takes a number and a callback. It has a tick
method that reduces the number by 1, and calls callback when it
reaches zero (and then resets the timer.)

Game uses it to repopulate the asteroids every few seconds.
Adds aimed asteroids every repop
Keeps track of sum of asteroid values

so that it doesn't have to be recomputed every time the field repops.

Also doubles the speed of fast asteroids.
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.
Removes unnecessary param, adds default values to randomPosition
Adds chance of weapon drop on asteroid breaking