Fix events/_item for events with no end time

We don't actually require end times on events, though our form strongly
encourages it. The recent update to add schema.org markup assumed that
end_time would always be present. This was a fairly safe assumption, 
given that we haven't noticed it in production until now. :)
Merge pull request #38 from reidab/fix_event_show_with_no_end_time

Fix events/_item for events with no end time