added support for adding an indicator to the calendar

I like your calendar picker.   It's clean, fast, easy to use and works well.   I'm working on a project that needs a calendar picker like this.   However, I need the ability to show an indicator on the calendar on a date by date basis, similar to how the iOS7 calendar app displays one.

I've added a new view to the cell that displays a tiny dot under the date and extended the delegate protocol.

I've also updated the demo app to show an indicator on every 4th day.