v3: Rework Brain

This turns brains into first class citizens in Hubot, not just a script. Hubot will call `robot.run()` when the brain emits a `ready` event. This event should be fired when data has been loaded into the brain and ready to be used.

A brain is basically an `npm` package named `hubot-brain-{something}` and has the old brain script as the entry point. So should be super simple to extract them out of `hubot-scripts`.