Fix NeoBundle URL in README and install script
install.sh: small refactoring

- Move creation of cache directory to the top
- Use variables to name the spell files
install.sh: don't get spell files if already downloaded
install.sh: add -f option to force downloads

When the option is passed, the bundle and spell directories will be
deleted, forcing the download of the plugins and the spell files.
install.sh: add -s option to skip plugins install
install.sh: improve command to install plugins

- Replace NeoBundleInstall! with NeobundleUpdate! to ensure that all
  plugins are installed / updated.
- Call Vim in silent mode. Run visual to fix the exit code.
- The new verbose option print some errors about files not found in the
  runtimepath that can be ignored.

[1]: https://github.com/Shougo/neobundle.vim/blob/master/bin/neoinstall