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

[twisteroid ambassador]

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