aggregate multiple event emitter events + data onto a single event emitter that emits data events on change.
var es =ee1 =ee2 =esee1esee2esee1 // since "one" and "two" are undefined, it'll clear those states
es.listen(emitter, eventName, ['list', 'of', 'bindings']) -> es
listen to an
eventName. when that emitter emits that event name,
it will update the state object. the state object will then emit a
containing the current state.