Show an event marker for particular dates

[percysnoodle]

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.

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.

[puls]

This is wonderful, but I'm a little hesitant to adopt anything that brings its own appearance to the party. Just a couple tweaks and it could be fine.

[pajai]

Hi guys,

Eager to see this functionality available on master. Could we go ahead and merge back these changes?