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/columns/CMakeLists.txt

@@ -21,6 +21,7 @@ set (columns_SRCS Column.cpp Column.h
                   ColPriority.cpp ColPriority.h
                   ColProject.cpp ColProject.h
                   ColRecur.cpp ColRecur.h
+                  ColScheduled.cpp ColScheduled.h
                   ColStart.cpp ColStart.h
                   ColStatus.cpp ColStatus.h
                   ColString.cpp ColString.h