Implement syncing of All-Day events to Google Calendar

Issue #85

Why is this change necessary?
- All day events are events entered by user into TaskBuddy that do not
  have start times and end times, but only start dates and/or end dates.
- TaskBuddy logic automatically fills up the timings to start with 0000
  and end with 2359. However, google calendar has a special flag for
  all-day events, and they appear in a visually different area on Google
  Calendar.
- Therefore it is important to store such events as all-day events in
  Google Calendar, rather than as a normal event which begins from 0000
  and ends at 2359.

How does this address the issue?
- This change fulfils important issue #85: To sync (add) All-Day events to
  google calendar.