Added a delegate callback to allow the picker to show activity on a given date.

Like the iOS calendar app:

![screen shot 2013-05-03 at 17 43 05](https://f.cloud.github.com/assets/34273/459981/f7f37e38-b410-11e2-929a-59b260fdf9e6.png)


Don't mind if you decide not to accept the Pull Request. DayFlow is perfect for something I'm working on, but needed this tiny tweak. I figured I'd send it back in case you want it.

Thanks!