Implement add method for new Database API

https://github.com/cs2103aug2014-f11-3j/main/issues/67

Why is this change necessary?
The add method for the new Database API (see link) no longer
accepts/returns a Bundle object for message passing. Instead, exceptions
are now thrown and a new add method is needed to handle/throw
appropriate solutions. Also, task IDs need to be reset after every add;
this needs to be incorporated into the new add method.

What side effects does this change have?
- Exceptions thrown from GoogleCalendarManager can now be handled
  appropriately.
- Dynamic task ID allocation means that display of task IDs to user has
  to be refreshed constantly to ensure accurate task manipulation.