Date Formatting

- Some bad inefficiencies in date formatting were noticed, and when addressed,
  caused a bug to surface.  The length of a formatted date can be calculated
  from the dateformat, but was done incorrectly.  Very, very incorrectly.
- Added unit tests.
- Promoted date column-specific "countdown" size measurements up to the ColDate
  base class.  This neatly falls out from work on #1218.
- Noted a potential I18N problem in Date.cpp.

src/columns/ColDue.h

@@ -39,8 +39,6 @@ public:
 
   bool validate (std::string&);
   void setStyle (const std::string&);
-  void measure (Task&, unsigned int&, unsigned int&);
-  void render (std::vector <std::string>&, Task&, int, Color&);
 };
 
 #endif