tell.coffee: Separate rooms, Nickname matching

Expand tell.coffee to separate rooms from each other. Otherwise, messages could end up in the wrong room or even with the wrong person if different people use the same username in different rooms.

The other new feature is case-insensitive prefix-matching for usernames. Some people use different username suffixes depending on the device they're on, but we still want to be able to tell them stuff as soon as they enter.

Also, send the messages when the user *enters* the room, not when they *say* something.

This is basically a merge of [this gist](https://gist.github.com/lorenzhs/5227272).