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/custom.html

@@ -88,6 +88,7 @@ report.mine.sort=priority-,project+</pre></code>
                   <li>age
                   <li>active
                   <li>tags
+                  <li>recur
                   <li>description
                 </ul>