Prepare for 0.4.0.M1

Docs tweaking and package info renaming in README.md
We need to define what it means for the Publisher–Subscriber (and possibly Subscription if one was created) for `onError` to throw, but defining the part of the behavior that is external to this relationship cannot meaningfully be covered by this spec—requiring to re-throw is only valid in certain scenarios, other implementations might have different mechanisms in place to deal with this. My proposal is:

> A failing `onError` invocation is a violation of the specification. In this case the Publisher MUST consider a possible Subscription for this Subscriber as canceled. The Publisher SHOULD raise this error condition in a fashion that is adequate for the runtime environment (e.g. by throwing an exception, notifying a supervisor, logging, etc.).