Pull Request - onBalloonClose

Hi jgilfelt,
It's my first time sending a pull request, so I'm not 100% sure that I did everything right.
I modified a couple of things in your BallonItemizedOverlay because I'm using it, and while extended, I felt the need of another method, "onBallonClose", which I think It may be useful to others.

So what I did, was:

Added method onBallonClose
Modified method onBalloonOpen for better consistency

Hope you liked it.