* created autoload/vundle/compat.vim: "compatibility" module
    (implements functions not available in previous vim versions);
    (code taken from my previous commit in 'autoload/vundle/config.vim',
    then modified and extended);
  * added compatibility function for shellescape();
 * autoload/vundle/config.vim: tidied up so that it's a bit closer to
    the file upstream;
  * (previous commit) but added a few performance improvements (avoid
     unnecessary calls and/or operations);
 * altered other scripts to use 'vundle#compat#*()' functions instead of
    those not necessarily available in previous vim versions;