Async review

Hi, since I need this crate for a project I'm working on, I decided to give implementing futures `Stream` for a BusReceiver (Bus sending isn't async in this patch), however I am curious as to whether this implementation is correct, since I'm not too experienced with lock-free code

[lib.rs](https://github.com/SethDusek/bus/blob/master/src/lib.rs#L597)

Basically, if a value is not ready, it sends the current Waker (kind of equivalent of thread handle) to the unparking channel, which will wake it up when it's ready.