Task Validation

- Fixed validation bug whereby recurring child tasks inherited the
  parent status ("recurring") instead of the expected "pending".
- Modified 'add' command to code duplicated in Task::validate.
- Cleaned up associated bug.period.t unit test.

src/TDB2.h

@@ -93,8 +93,8 @@ public:
   ~TDB2 ();
 
   void set_location (const std::string&);
-  void add (const Task&);
-  void modify (const Task&);
+  void add (Task&);
+  void modify (Task&);
   void commit ();
   void synch ();
   void revert ();