start project4.
for project4
1. add the UML class diagram, in file 'sicp-project4-unl-class.dia'
2. add the map of the world, in file 'sicp-project4-map.dia'
3. fix the bug for procedure populate-spell

4. now, I am fixing the bug for error message "I don't know how to make a
   handler for #f" from procedur ->handler.
nothing important, add something to readme.txt