Upgraded resolution

The old resolution was 352x352 so that people with older monitors could play, but I figured that's really a lot lower than most people would ever need, so I bumped it up to 704x704.

I'll probably end up using different sprites for the cave and the character, these don't look that good when each cell is 64x64. I've had a couple of offers from people to do graphics for me, so hopefully there'll be something better looking down the road.