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/parse.cpp

@@ -475,14 +475,6 @@ void parse (
     }
   }
 
-  if (task.getAttribute ("recur") != "" &&
-      task.getAttribute ("due")   == "")
-    throw std::string ("You cannot specify a recurring task without a due date.");
-
-  if (task.getAttribute ("until") != "" &&
-      task.getAttribute ("recur") == "")
-    throw std::string ("You cannot specify an until date for a non-recurring task.");
-
   if (validDescription (descCandidate))
     task.setDescription (descCandidate);
 }