Handle event bubbling

[arboleya]

Something like:

var happens = require('happens');

var a = happens();
var b = happens().bubble(a);

a.on('change', function(){
   console.log('changed a');
});

b.on('change', function(){
   console.log('changed b');
});

b.emit('change'); // 1) change b 2) changed a

There needs to be a way to stop the bubbling at any time though. No event (instance) param should be passed along any event handler, for clean signature.