changed sprite to png format, added collision detection for two balls
created a group of enemy balls which can collide into each other.
hit detection now works
playerball can kill enemy ball on hit
added animation to show collision. However there is still a bug where if two hit events happen in a short time the playerBall stays large
collisions animated, added some functions that will be used in future iterations
Merge pull request #2 from johnlok/animate_collision

Animate collision