Integration - TDB write operations

- TDB::gc rewritten.
- TDB::commit fixed.
- Corrected usage of handleRecurringTasks wrt TDB.
- Unit tests for TDB.

src/TDB.h

@@ -53,11 +53,11 @@ public:
   int   loadPending (std::vector <Task>&, Filter&);
   int   loadCompleted (std::vector <Task>&, Filter&);
 
-  void  add (Task&);             // Single task add to pending
-  void  update (Task&, Task&);   // Single task update to pending
-  int   commit ();               // Write out all tasks
-  void  upgrade ();              // Convert both files to FF4
-  int   gc ();                   // Clean up pending
+  void  add (const Task&);    // Single task add to pending
+  void  update (const Task&); // Single task update to pending
+  int   commit ();            // Write out all tasks
+  void  upgrade ();           // Convert both files to FF4
+  int   gc ();                // Clean up pending
 
 private:
   FILE* openAndLock (const std::string&);