Support for event dates

Hi, in case you're interested I've added support for event dates, which are highlighted with a different background colour and added to a calendar view using `[calendar addEventAtDate:aDate]`.

![Screen Shot 2013-02-19 at 10 45 16](https://f.cloud.github.com/assets/1949/170912/80406d88-7a81-11e2-8df6-33233477d9c2.png)