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/T.h

@@ -56,7 +56,7 @@ public:
 
   const std::string getDescription () const            { return mDescription; }
   void setDescription (const std::string& description) { mDescription = description; }
-  int getAnnotationCount () const                     { return mAnnotations.size (); }
+  int getAnnotationCount () const                      { return mAnnotations.size (); }
 
   void getSubstitution (std::string&, std::string&) const;
   void setSubstitution (const std::string&, const std::string&);