Show an event marker for particular dates

This adds a method to the calendar delegate, `-calendarView:shouldDisplayEventMarkerForDate:`, which allows the delegate to specify that a marker should be displayed under the number.

![screen shot 2013-05-10 at 15 11 06](https://f.cloud.github.com/assets/301674/488312/7e7674f0-b97b-11e2-8f8a-068e89eeb948.png)

The marker is the bullet character '•' so no additional images are required, and the delegate method is optional so this shouldn't break any existing installations.