Merge pull request #6 from s-osa/feature/3_jan_parser

Feature/3 jan parser　#3
Jan::CheckDigitCalculator #5
Jan::Parser.body #5
Refactoring. #5
Convert classes to modules. #5
Use 'check_digit' instead of 'checkdigit'. #5
Merge pull request #7 from s-osa/feature/5_jan_check_digit_calculator

Feature/5 jan check digit calculator　#5
Jan::CheckDigitCalculator returns correct answer in 0 check digit case. #4
Jan::Random.code(size) #4
Jan::Parser.instore_code? and Jan#instore_code? #4
Jan::Random.instore_code(size) #3