z is a bash/zsh extension that tracks your most used directories,
based on 'frecency'. After  a  short  learning  phase, z will take 
you to the most 'frecent' directory that matches ALL of the regexes 
given on the command line, in order. For example, z foo bar would 
match /foo/bar but not /bar/foo. Frecency is a portmanteau of 
'recent' and 'frequency'.
