Migrate from Java Nio to Netty

Currently, Kryonet uses the nio library of java. It does its job, but, In my opinion, it suffers from poor maintainability and performance. If we migrated to Netty, we would get
- better, more maintainable code
- better performance, especially on Linux or Mac using native transport

In the next few weeks, I'll look into it and submit a pull request. In the mean time, I would appreciate if we could discuss this topic here.