updated todo
added spark particle shape
gave emitter concept of dying
gave VisibleGameObject concept of dying
got rid of dummy emitter
game object manager now culls dead objects
game ball now sparks on collision with wall
added ctor without arguments to ParticleShape
got ripple shader working
added background image to show ripple shader
play screen uses background image and ripple shader
spark now takes a direction
spark now takes a direction arg, also changed the update to look a little more like sparks