Moves weapon bullet stats into its own object

Weapon structure now looks like:
  cooldown
  speed
  bullet
    radius
    damage
    color
    lifetime
Realigns 'edge flip' point for moving objects
Adds onScreen instance var for movingobjects
Places initial asteroids off screen

Adds new MovingObject method, placeOffScreen, which determines the
object's general direction (horizontal or vertical) and places it
just off the proper edge of the canvas so that it will float into
view. Asteroid::randomAsteroid calls this on the asteroid it makes.