Umegaya/feature/emitter

- implement emitter
 - not callback but resume coroutines which is wait for emit
- 2 event handle primitive
 - select : wait until one of events are emit
 - wait : wait until all of events are emit
- implement tentacle
 - it fiber but emit 'end' when its finish
 - combine tentacle and select, wait, you can describe complex event waiting code easily