Support @<name> convention in the Robot's respond listener

Currently, a Robot (named Hubot, for example) will only respond to messages that begin with Hubot. But Tooter and Ferceberk conventions have people in the habit of using @ convention within Campfire (and other) chats, and even clients like Flint support this usage for name completion. This simple updates makes the Robot @-friendly. And who doesn't like a friendly robot?