Recurring Tasks - upgrades and bug fixes

- Improved (fixed) logical consistency checks that prevent the
  addition of a recurrence frequency without a due date.
- Improved (fixed) logical consistency checks that prevent the
  addition of an until date with a recurrence frequency.
- When a recurring task is modified, all sibling instances, as well
  as the parent task now get modified.
- When a recurring task is appended, all sibling instances, as well
  as the parent task now get modified.
- Updated documentation.
- It is now possible to upgrade a regular task to a recurring task,
  which is triggered by the "recur" attribute.

src/task.h

@@ -89,6 +89,7 @@ std::string handleStop (TDB&, T&, Config&);
 std::string handleUndo (TDB&, T&, Config&);
 std::string handleColor (Config&);
 std::string handleAnnotate (TDB&, T&, Config&);
+T findT (int, const std::vector <T>&);
 
 // report.cpp
 void filter (std::vector<T>&, T&);