Report date format

- added new reportdateformat to extend the formatting of due dates
  in the reports and "task info"
- added new conversion sequences a, A, b, B and Y to be used with
  reportdateformat

src/Date.h

@@ -58,6 +58,7 @@ public:
   static std::string dayName (int);
   static int weekOfYear (const std::string&);
   static int dayOfWeek (const std::string&);
+  static int monthOfYear (const std::string&);
 
   int month () const;
   int day () const;