Merge actress into concurrent-ruby

followup of #66

- [ ] Unnecessary method lookup cache dropping, see [source](https://github.com/jdantonio/concurrent-ruby/blob/master/lib/concurrent/actor/simple_actor_ref.rb#L20), originaly #67
- [ ] make sure all `IVar`s are resolved when actor is shutdown before processing the message
- [ ] merge actress into concurrent-ruby

**Later**
- [ ] un/become ?
- [ ] dead letter routing ?