Make the physics and difficulty more like Flappy Bird.

I think this makes the physics feel more like Flappy Bird. Mostly by just adding resistance. Try it!

Also this lets me get like a score of 5 (instead of a high of 1), which is pretty much my average score for Flappy Bird proper. #sad

![ios simulator screen shot feb 22 2014 9 59 45 pm](https://f.cloud.github.com/assets/504184/2239762/4da7e1fc-9c50-11e3-962d-17f959cdaf58.png)