Prepare for 0.4.0.M1

Docs tweaking and package info renaming in README.md
So who calls `Subscription.cancel()` if an error occurs in an `Operator` in the middle of a chain to tell the upstream to shut down?

We need to ensure that `Subscription.cancel()` always gets called upwards after a terminal event, as there is no guarantee it was the origin that originated the `onError` or `onComplete`.