Add async support

[gulachek]

There are lots of async libraries. Add support for an implementation-agnostic way of doing catui operations. These would be documented so that someone using an async library like boost asio or libuv which manage descriptors could very easily hook this up to their runtime

[gulachek]

Almost there. Now that libraries are merged, should add encode/decode for all messages/communication. This will allow easy wrappers for the async lib being used. Already have some support for server encoding side for ack/nack, but still need connect, decode ack/nack, etc.