Prepare for 0.4.0.M1

Docs tweaking and package info renaming in README.md
In fact that should really be in the `Subscription`. But otherwise what kind of limit shall we define ? I'm not a huge fan of signaling onError that would prevent calling code to deal with it directly. Maybe rising an IllegalStateException ? In the other hand that's a case where having an API defined exception could be handful as this a possible signal for multiple things: overflowing to a journal etc).