Save username for both user.name and user.id

We're currently saving the irc nick as `user.name` and using the unix time of the first user sighting as the `user.id`. Authentication with auth.coffee assumes that `user.id` is unique in hubot's brain, and something that someone can store in the `HUBOT_ADMIN_USERS` envvar.

**I think IRC nick is the best `user.id` we have, and we should use that.** Yes, it isn't always unique, but we need to assume so in order to get hubot-irc working with auth.coffee. The actual auth will happen when a privileged command is called, so if the person isn't identified with nickserv, that service will let us know at that time.

Any objection? As far as I can tell, the timestamp doesn't really accomplish anything except ensuring that there's never a user id collision (which is unlikely because we're talking seconds, but it's technically just as inappropriate as using UTC rounded to the hour or minute :)

Thoughts?

Related: #103