Bug #417 - Sorting by countdown_compact not working

- Added support for more varied durations when specifying recurring tasks,
  such as '3 mths' or '24 hrs'.
- Fixed bug #417, which caused sorting on countdown fields to be wrong
  (thanks to Michell Crane).
- Durations are now based on seconds, rather than days, and can accept/parse
  negative durations.

src/Duration.h

@@ -52,6 +52,7 @@ public:
 
 private:
   time_t mSecs;
+  bool mNegative;
 };
 
 #endif