begin chapter 15 objects
complete 15.1
finish 15.2 Attributes
finish 15.3-Rectangles
finish 15.4-Instances as return values
finish 15.5-Objects are mutable
finish 15.6-copy
finish 15.7-Debugging
finish 15.8-Glossary
类型对象 -> 类对象
finish chapter 15. Classes and objects
fix a mistake
fix a mistake
fix some mistakes in chap.15
Merge pull request #12 from endyul/master

finish chapter 15. Classes and objects