VIM: Use Vundle to manage plugins.

* NeoBundle was useful with unite.vim and vimproc.vim, but I don't use
  them anymore.
* Vundle is simpler and gets the job done.
Make the system ready (vim included) with scripts/install.

* Install Vundle.vim with script/install.
* Cleanup code in scripts/{install,remove} files.
* Update README.