improved compatibility: it now supports vim version 7.0

 * added new functions to detach caller from having to call
    fnameescape() (which isn't available until vim-7.2):
  * s:compat_rtp_rm_entry(dirs): abstracts 'set rtp-=LIST';
  * s:compat_rtp_addset_entry(dirs, addset_operator):
     abstracts 'set rtp+=LIST', 'set rtp^=LIST',
     'set rtp=LIST';