Add Rakefile with install and uninstall tasks

* Run `rake install` to create symlinks in $HOME that point to each
  dotfile with a .symlink suffix
* Run `rake uninstall` to remove those symlinks
* Currently still requires a manual installation of vundle:

    git clone git@github.com:gmarik/vundle.git ~/.vim/bundle/vundle
    vim
    :BundleInstall