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.

html/task.html

@@ -123,6 +123,8 @@
                       to help ensure higher quality releases.
                   <li>Fixed bug that caused large performance hit during table rendering.
                   <li>Fixed bug that concatenated a modified description without spaces.
+                  <li>Added new column 'recur' that displays the recurrence period of any
+                      recurring tasks.  This column can be added to any custom report.
                 </ul>
 
                 <p>