Implement add method for backward sync

https://github.com/cs2103aug2014-f11-3j/main/issues/122

Why is this change necessary?
- The current add method automatically syncs the added task to Google
  Calendar.
- However, when Google Calendar manager wants to add a task to database
  during its backward sync, the forward sync should not occur.

How does it address the issue?
- A new add method, which is similar to the normal add method but
  without the forward sync to Google Calendar, solves this problem.