Some changes

* Make mdx_ultisnips a separate repository in my github, and use
  neobundle to manage it as a normal plugin.
* Adjusted the configuration of 'Ultisnips' to accommodates the
  changes above.
* Renamed neobundle bundle to replace all '-'s with '_'s.
* Used 'g:vim_config_root' as much as possible in vimrc to remove
  inconsistencies between 'linux-neobundle/vimrc' and
  'win32-neobundle/vimrc'