mostly about the last pull request

Sorry - I meant to add more comments to the last pull request. There are a few changes in there that are just suggestions - you might not want to include them.

I thought it looked a bit funny having the asteroid belt going round the base, so
increased radius of asteroid belt (and number of asteroids in it so that they don't get too sparse - maybe this slows it down though?) so the player can't immediately see what is at the centre of the belt  

also a few other things:

 * started ship off at 0,0 so that the position of everything else was easier to work out

 * added some more variables to spawnAsteroid, to make it clearer

 * added a Saturn (tiny picture, so assumes it's far away i.e. out of plane of asteriod belt?). Placed so that it will come into view if user drifts straight past asteroid belt, and will help them see if they're using the thrusters. (radius=0, mass=0)