Fix error in chapter8
Merge pull request #55 from zyuanhong/add_sth

Fix error in chapter 8
fix link
Merge pull request #56 from lonre/fix-chapter6-link

fix link
Fix typo and indentation in chapter8
Merge pull request #57 from zyuanhong/add_sth

Fix typo and indentation in chapter8
fix a typo error and add a missed snippet of code
Merge pull request #58 from logchi/gh-pages

Fix a typo error and add a missed snippet of code