Remove generated auth.coffee, and use external hubot-auth instead.

This is another take on https://github.com/github/hubot/pull/656 . It removes auth.coffee from `src/scripts`, so it won't be included in newly generated bots. Instead, it adds `hubot-auth` to `package.json` and `external-scripts.json`.

Tested this briefly, and seems to work as expected.

My only concern at the moment has to do with `package.json` entry. This should be flexible, but we might need to be updating the minimal version as hubot-auth, which wouldn't be great. It might be better to do something like `npm install hubot-auth --save`, so it'll grab the latest version and lock it at a particular minor version (ie `~1.1.2`). That might take a bit more doing than we want to in `creator.coffee` though.