Bug #542

- Fixed bug #542, which sorted the countdown columns incorrectly (thanks to
  Michelle Crane).
- Duration could not parse all varieties of it's own output, which is why the
  sorting was broken, in particular negative durations and real numbers in
  durations (such as (1.2y').
- Sorting in Table::sort_compare for Table::periodAscending and
  Table::periodDescending was broken.
- Missing unit tests for countdown_compact.
- Imported Nibbler::getNumber from the Attitash project.
- Additional Duration unit tests.
- Additional Nibbler unit tests.
- Additional countdown.t unit tests.

src/Nibbler.h

@@ -50,6 +50,7 @@ public:
   bool getQuoted (char, std::string&, bool unescape = true, bool quote = false);
   bool getInt (int&);
   bool getUnsignedInt (int&);
+  bool getNumber (double&);
   bool getLiteral (const std::string&);
   bool getRx (const std::string&, std::string&);