unicode string test, encode and decode
exmaple 6.3 using lists as a stack; fix chapter02.py menu item -> specific function.
ex 6-2: string identifiers: idcheck.py version 2.0
ex 6-3: sorting
exercises up to ex 6-8, code format
ex 6-14: "rock, paper, scissors" game
ex 6-15: days lapse between two dates
fix daylapse: days between two dates