Merge pull request #4 from xiehuc/master

finish chapter inheritance
fix a tiny bug
complete chapter 14
complete chapter 14
Merge pull request #5 from endyul/master

complete chapter 14
finish method chapter section 13

Signed-off-by: xiehuc <xiehuc@gmail.com>
Merge branch 'master' of github.com:carfly/thinkpython-cn