Recurring Tasks

- Rewrote updateRecurrenceMask, which was hopelessly muddled after the
  TDB -> TDB2 change.
- Modified associated code.
- Improved signal to noise ratio in unit tests.

src/main.h

@@ -41,7 +41,7 @@
 void handleRecurrence ();
 Date getNextRecurrence (Date&, std::string&);
 bool generateDueDates (Task&, std::vector <Date>&);
-void updateRecurrenceMask (std::vector <Task>&, Task&);
+void updateRecurrenceMask (Task&);
 int getDueState (const std::string&);
 bool nag (Task&);