add credits for wooden background
add NSCoding support to GoBoardPosition (fixes defunct bug report feature)
update UML class diagram with core classes
fix memory leaks (most of them UIBarButtonItems not autoreleased)
remove "undo" icon that is no longer needed
add more logging

change details:
- use logging in all commands
- use logging in almost all cases of assert() (except in table view controllers)
- use logging where exceptions are raised
- use logging in many other error cases
set logging to rotate by file size, to use 1 MB per file, and to use 10 rotations
overhaul of MANUAL document
update doxygen target to use the MacPorts path /opt/local/bin
known bug: resigned state of a game is lost when a game is saved or loaded
bug report section is no longer disabled when computer player resigns

remove unnecessary optimization that caused the "Bug report" section to become
permanently disabled when the computer player resigned