Created new class: GoogleCalRetriever

This new class will be used to handle retrieving of Google Calendar events
from Google Calendar.

As of v0.2, retrieving from Google Calendar does not have many uses
besides testing purposes. In later revisions, backward sync may require
its use more.
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.