Feature: scheduled date

- Supports 'scheduled' date for tasks, which represent the earliest opportunity
  to work on a task.
- Added unit tests for urgency.
- Implemented color rule.
- Added scheduled to 'info' report.
- Updated assorted documentation.
- Removed comments in default rc because they are never seen.
- Added (broken) 'ready' report, which is like 'next' but only for ready tasks.

src/Task.h

@@ -110,10 +110,12 @@ public:
 private:
   int determineVersion (const std::string&);
   void legacyParse (const std::string&);
+  void validate_before (const std::string&, const std::string&);
 
   inline float urgency_priority () const;
   inline float urgency_project () const;
   inline float urgency_active () const;
+  inline float urgency_scheduled () const;
   inline float urgency_waiting () const;
   inline float urgency_blocked () const;
   inline float urgency_annotations () const;