Added support for intervals every x weeks

This adds support for weekly intervals e.g. every second or third week. You should be able to use it like this:

var aEvent = new Event()
            {
                Frequency = 2,        // weekly
                DaysOfWeek =  4,      // every Tue
                WeeklyInterval = 2, // every 2nd week
                FirstDateTime = new DateTime(2013, 1, 1)  // start date of the event
            };

Thought you would like this. :)