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.cpp

@@ -73,46 +73,3 @@ void ColumnDue::setStyle (const std::string& value)
 }
 
 ////////////////////////////////////////////////////////////////////////////////
-// Set the minimum and maximum widths for the value.
-void ColumnDue::measure (Task& task, unsigned int& minimum, unsigned int& maximum)
-{
-  minimum = maximum = 0;
-
-  if (task.has (_name))
-  {
-    if (_style == "countdown")
-    {
-      Date date ((time_t) strtol (task.get (_name).c_str (), NULL, 10));
-      Date now;
-      minimum = maximum = Duration (now - date).format ().length ();
-    }
-    else
-      ColumnDate::measure (task, minimum, maximum);
-  }
-}
-
-////////////////////////////////////////////////////////////////////////////////
-void ColumnDue::render (
-  std::vector <std::string>& lines,
-  Task& task,
-  int width,
-  Color& color)
-{
-  if (task.has (_name))
-  {
-    if (_style == "countdown")
-    {
-      Date date ((time_t) strtol (task.get (_name).c_str (), NULL, 10));
-      Date now;
-
-      lines.push_back (
-        color.colorize (
-          rightJustify (
-            Duration (now - date).format (), width)));
-    }
-    else
-      ColumnDate::render (lines, task, width, color);
-  }
-}
-
-////////////////////////////////////////////////////////////////////////////////