Fix ics output to do all-day and multi-day events correctly.

RFC 2445 (section 4.6.1) specifies that a VEVENT end date is
non-inclusive, therefore for all-day or multi-day events, the end date should be the
following day.
Merge pull request #381 from alphagov/fix_ics_output

Fix ics output to do all-day and multi-day events correctly.