Updated version of reference implementation as a standalone module
    * changed interface: which_files() returns generator, which() returns first match,
      or raises IOError(errno.ENOENT)
    * updated doctest

Made this to more closely resemble the 'which' command behavior, and to make the typical use case simpler. The generator interface is still a good idea imho, so it's kept as which_files(). I'm already using the reference implementation module flawlessly, so I don't see any other changes needed on my side. Your ideas are welcome of course.