Fix parsing of VEvent without UID

If a feed has no UID (like this one: http://royalsociety.org/eventsiCal.aspx), a notice is throw:

      [ErrorException]                                                                                                       
      Notice: Undefined index: uid in /Users/dunglas/workspace/msw/vendor/intouch-iCalendar/intouch/ical/VEvent.php line 50