Merge pull request #36 from perfectfoolish/gh-pages

fix typo in ch3
fix error
fix typos in chapter 4, 5, 6, 7, 8
Merge remote-tracking branch 'origin/gh-pages' into gh-pages

* origin/gh-pages:
  fix an error
Merge pull request #37 from phaibin/gh-pages

fix typos in chapter 9
Merge remote-tracking branch 'origin/gh-pages' into gh-pages

* origin/gh-pages:
  fix error