Remove auth.coffee from hubot core

EDIT: Original issue title = Clarify where auth.coffee lives

This script seems to live in two places with very different histories:
https://github.com/github/hubot/commits/master/src/scripts/auth.coffee
https://github.com/github/hubot-scripts/commits/master/src/scripts/auth.coffee

Can we get these in sync or kill the one in the hubot repo proper? Really seems like something that should live in community scripts (and extracted into it's own repo later). I get that it's nice to bundle things like this with hubot, but it strikes me as an evolving script that should live where it can easily evolve (ie. not copy-pasted during hubot creation) :)