Authen::PhoneChallenge

This module is a very simple challenge/response system that uses strictly
numeric data ([0-9]).  The original intent was for use in a automated phone
system.

User dials extension that requires authentication for some reason.
User asked for their identification number.
User given a challenge number.
User looks up the proper response to the challenge on previously issued card and enters it.
Call continues as normal.

The idea was inspired by OPIE, but is in no way to be considered as secure.
Additional logic such as maximum failure counts, etc are left as an exercise to
the calling code.
