Installation:
  git clone git://github.com/yiningzhao/dotvim.git ~/.vim

Create symlinks:
  ln -s ~/.vim/vimrc ~/.vimrc
  ln -s ~/.vim/gvimrc ~/.gvimrc

Switch to the `~/.vim` directory, and fetch submodules:
  cd ~/.vim
  git submodule init
  git submodule update

Install new plugins:
  git submodule add http://github.com/tpope/vim-fugitive.git bundle/fugitive
  git add .
  git commit -m "Install Fugitive.vim bundle as a submodule."

Upgrade a plugin
  cd ~/.vim/bundle/fugitive
  git pull origin master

Upgrade all plugins
  git submodule foreach git pull origin master