Recurring events

Added support for recurring events.

Works like this:

    event = cal.create_event do |e|
      e.title = 'Work-day Event'
      e.start_time = Time.now
      e.end_time = Time.now + (60 * 60) # seconds * min
      e.recurrence = {freq: "weekly", byday: "mo,tu,we,th,fr"} 
    end

I created some tests as well. If you have any comments please let me know.