ruby module that emulates the 'which' program

 Searches the path, or a provided path with given separators (path_sep,
 commonly “:”) and a separator for directories (dir_sep, commonly “/”), will
 return all of the places that filename occurs. `filename' is included as a
 part of the output.
 .
 Note that the filename must both exist in the path and be executable for it to
 appear in the return value.