[Async-sig] async_stagger: Happy Eyeballs implementation in pure asyncio

Repo: https://github.com/twisteroidambassador/async_stagger Docs: http://async-stagger.readthedocs.io/en/latest/ Provides near drop-in replacements for open_connection() and create_connection() using Happy Eyeballs. Also exposes the underlying scheduling logic where you can plug in your own coroutines to run. I basically ported trio's implementation to asyncio, and it turned out to be not too difficult. Cheers, twistero