TDB2

- TDB2::add no longer assumes that adding one task results in only one task
  begin added, as hook processing may expand the set.
TDB2

- Combined add and modify operations in ::add, in case the hook processing
  generates modifications.
TDB2

- Consolidated most of the processing in ::add and ::modify into ::update.
Hooks

- Stubbed new ::onAdd and ::onModify interfaces.
Hooks

- Implemented new interface.  Testing time.