Fix score writing

There was an error when writing a larger score
ex: 0 --> 10 or 100 --> 1000
Remove unneeded images
add gun upgrade

doesn't spawn yet
bad image
lets you get negative coin value
Fix reset button
Using darker polar bear again.
Don't track .git
Always show the highscore text

it wouldn't show up on first startup of the game
GunUpgrade changes

shotDelay now counts up instead of down. You can shoot if it's greater
than delayMax. GunUpgrade now changes this variable directly,
eliminating the need for extra methods.

SnowMobiles go farther offscreen, so their tracks don't disappear so
suddenly.

SnowMobile explosion should work properly on Macs now.