New Column - recur

- Added new column 'recur' for use in custom reports.
- Implemented Table::ascendingPeriod, Table::descendingPeriod allowing
  sorting on the recur column.
- Added unit tests to both use the new column and test the sorting.
- Code cleanup.

src/task.h

@@ -126,7 +126,7 @@ void formatTimeDeltaDays (std::string&, time_t);
 std::string formatSeconds (time_t);
 const std::string uuid ();
 const char* optionalBlankLine (Config&);
-int convertDuration (std::string&);
+int convertDuration (const std::string&);
 std::string expandPath (const std::string&);
 
 #ifdef SOLARIS