Game scene enhancements

[christopherbot]

• power ups - invincibility, freeze opponent, speed up, bigger snake, next ball hit sends 2 balls back, warp thru walls, etc.
• when you hit a wall, remove the head of the snake so that if you keep running into the wall you'll continue to lose snake pieces Wrapping through the walls has proven to be pretty fun so I'm not going to do this
• secrets: eat your tail, get a power up?