Make HTTP async as well with async-http

[dblock]

Coming from https://github.com/slack-ruby/slack-ruby-client/issues/210#issuecomment-416109261

One thing to consider is that you also want APIs like chat_postMessage to be async. For my hack around the other slack gem, I just made a wrapper for faraday which worked fine. I suggest you continue with the next release supporting different models for concurrency. However, on your next major release (0.13), I suggest you go all in, removing EM, Celluloid, etc, and using async-http and async-websocket. You can get concurrency across all the APIs.