* bash-like pushd, popd, dirs

* see http://www.gnu.org/software/bash/manual/bashref.html 

    section 6.8.1 Directory Stack Builtins

* see samples/*.rb


the main differences are that backward counting starts from -1 in typical ruby 
fashion, not -0 and that options are specified using symbols, eg.

  Dir.pushd(Dir.pwd, :n)