git clone vs checkout vs pull
http://blog.mikepearce.net/2010/05/18/the-difference-between-git-pull-git-fetch-and-git-clone-and-git-rebase/
http://qiita.com/ota42y/items/e082d64f3f8b424e9b7d
http://qiita.com/osamu1203/items/cb94ef9da02e1ec3e921
http://stackoverflow.com/questions/292357/whats-the-difference-between-git-pull-and-git-fetch