No more placeholders!

I have added the ground sprite and made the enemies explode when hit, the enemy robots also trundle around randomly or stand still looking angry.

I would now like to add an in-game UI to show score and such whilst playing, and then add a couple more enemy types before trying to dial in the physics and gameplay a bit.

I think the game balance will probably take a bit of work to get right, as there are multiple parameters that can be tweaked to change the feel and difficulty of the game, such as the enemy density, how much you slow down when hitting the ground, and what kind of powerups you have purchased.