Adding hit() to GameObject

Not sure if the code could be better. But since there's touching() and touching(String str) I suppose it would only be normal to have a method hit() next to hit(String str) too.

Case: e.g. when multiple objects fall on multiple objects, on impact dust clouds will appear.

Just wanted to say that whenever I propose a commit, you should feel free to merge or not. I'll leave it to someone with more experience to make the call.