Update to hackney 0.10.0 (thanks to @benoitc)

hackney 0.10.0 can now send async response to a specific process

https://github.com/benoitc/hackney/commit/2731c0d6fc101f3ea801a3821dad9194ff29679d
Add async request to HTTPoison
Update README.md
Merge pull request #4 from edgurgel/async-request

Async request