*  Kefir.multiBus - Bus that can handle multiple(custom) event types

Could you add this commit? It adds support for custom events (not only VALUE and END). And also adds Kefir.multiBus.

The example:
```
var bus = Kefir.multiBus();
bus.on('custom', function(val) { console.log(val) });
...
bus.emit('custom', { data: 'test' });
bus.end();
```

onLog function also tracks custom events values.