Enhancement/Fixes - Duration

- Reorganized Duration::valid and Duration::parse to use the same list
  of supported constructs.
- Added missing copy constructor.
- Added missing ctor initializer list.
- Corrected handling of negative time_t values.
- Added support for more duration formats.
- Corrected autoComplete use.
- Added Duration::negative.
- Corrected unit test descriptions.

src/util.cpp

@@ -195,8 +195,8 @@ int autoComplete (
       }
 
       // Maintain a list of partial matches.
-      if (length <= item->length () &&
-          partial == item->substr (0, length))
+      else if (length <= item->length () &&
+               partial == item->substr (0, length))
         matches.push_back (*item);
     }
   }