polar bear panic demo day

Rather than constantly chaining "game.physics.arcade" think about making it a variable - game is already a global it looks like, so you could declare something like ```gamePhysics = game.physics.arcade``` and simplify the rest of your calls. It makes the code readable, and also allows a quick switch to a different physics model later if you need to do that (you only change one place.) Remember the don't repeat yourself rule wherever you can.