chp5 2th edition
3/4 finished.
ch5 31-36
update 3v
Page 55-59 of chp7 done by Young
Merge pull request #21 from yan9yu/gh-pages

Page 55-59 of chp7 done by Young
Merge pull request #19 from hanlu-chen/develop

ch5 31-36
Merge pull request #20 from dada889/gh-pages

update 3v
Merge pull request #18 from linsidu/develop

Develop
Merge branch 'gh-pages'
Merge pull request #22 from kfeng123/develop

chp1 not complete
3/4
Merge pull request #23 from flymark2010/develop

v2
Update task.md
almost complete.
completed.
completed.
Merge pull request #24 from flymark2010/develop

chap2 completed.
内容翻译完了
Merge branch 'develop' of https://github.com/kfeng123/data-science into develop