Lesson descriptions loaded from the database

* TimetabledLesson#description is now implemented and (using planted
  data) tested.
* LessonDescription.find_by_schoolday_and_lesson
* DataMapper::Property::SchoolDay implemented to map SchoolDay objects
  to strings in the database.
* Command::EnterLesson -> DescribeLesson (and implemented)
* At commandline, can run "sr 10 ..." instead of "sr enter 10 ..."
* At commandline, dates can be specified: "sr 10 yesterday ..."
* (Note: commandline code not tested yet, even interactively.)
* Database.current (to support LessonDescription; general use not
  encouraged).
* Database#schoolday!(str) -- raises error if not a schoolday.

doc/devel-log/2012-02-21-database-timetabled-lessons.markdown

@@ -217,3 +217,12 @@ improved the situation by raising an appropriate error message in that
 situation. Tests are passing now.
 
 I declare Database#timetabled\_lesson working and am committing the code.
+
+    Database#timetabled_lessons tested and working
+
+    * Couple of changes to the code of the method itself.
+    * Lesson and Obstacle got to_s(:brief).
+    * Error raised if obstacle config contains non-school day.
+    * TimetabledLesson got to_s, enabling easier testing.
+    * test/database.rb contains the tests for timetabled_lessons.
+    * Obstacle config got a new entry with lessons "12(3), 10(2)"