refactor event span/seg slicing methods to be more robust
refactor event span/seg slicing methods to be more robust

blubberly/fullcalendar,fullcalendar/fullcalendar,kleisauke/fullcalendar,rgnevashev/fullcalendar,avanti-technologies/fullcalendar,arshaw/fullcalendar,laoneo/fullcalendar,kleisauke/fullcalendar,deepskyr/fullcalendar,blubberly/fullcalendar,oleg-babintsev/fullcalendar,blubberly/fullcalendar,fullcalendar/fullcalendar,laoneo/fullcalendar,deepskyr/fullcalendar,arshaw/fullcalendar,kleisauke/fullcalendar,arshaw/fullcalendar,ouiptik/fullcalendar,rgnevashev/fullcalendar,oleg-babintsev/fullcalendar,MeteorAdminz/fullcalendar,arshaw/fullcalendar,avanti-technologies/fullcalendar,avanti-technologies/fullcalendar,fullcalendar/fullcalendar,ouiptik/fullcalendar,rgnevashev/fullcalendar,kleisauke/fullcalendar,deepskyr/fullcalendar,avanti-technologies/fullcalendar,ouiptik/fullcalendar,fullcalendar/fullcalendar,MeteorAdminz/fullcalendar,pietervisser/fullcalendar,rgnevashev/fullcalendar,blubberly/fullcalendar,ouiptik/fullcalendar,pietervisser/fullcalendar,MeteorAdminz/fullcalendar,deepskyr/fullcalendar,laoneo/fullcalendar,oleg-babintsev/fullcalendar,pietervisser/fullcalendar,MeteorAdminz/fullcalendar