TDB2 Merge

- Migrated TDB::merge to TDB2::merge.
- Removed obsoleted code from TDB.
- Modified unit tests for merge to use export instead of export.csv.

src/TDB2.h

@@ -97,6 +97,7 @@ public:
   void modify (Task&);
   void commit ();
   void synch ();
+  void merge (const std::string&);
   void revert ();
   int  gc ();
   int  next_id ();