Pykka

- Add `pykka.__version__` to conform with PEP 396. This deprecates `pykka.VERSION` and `pykka.get_version()`.

- Add `pykka.ActorRef.tell()` method in favor of now deprecated `pykka.ActorRef.send_one_way()`.

- Add `pykka.ActorRef.ask()` method in favor of now deprecated `pykka.ActorRef.send_request_reply()`.

- `pykka.ThreadingFuture.set()` no longer makes a copy of the object set on the future. The setter is urged to either only pass immutable objects through futures or copy the object himself before setting it on the future. This is a less safe default, but it removes unnecessary overhead in speed and memory usage for users of immutable data structures. For example, the Mopidy test suite of about 1000 tests, many of which are using Pykka, is still passing after this change, but the test suite runs approximately 20% faster.

- 10x speedup of traversable attribute access by reusing proxies.

- 1.1x speedup of callable attribute access by reusing proxies.

- Change and document order in which `pykka.ActorRegistry.stop_all()` stops actors. The new order is the reverse of the order the actors were started in. This should make `stop_all()` work for programs with simple dependency graphs in between the actors. For applications with more complex dependency graphs, the developer still needs to pay attention to the shutdown sequence. (Fixes: 8)

- If an actor that was stopped from `pykka.Actor.on_start()`, it would unregister properly, but start the receive loop and forever block on receiving incoming messages that would never arrive. This left the thread alive and isolated, ultimately blocking a clean shutdown of the program. The fix ensures that the receive loop is never executed if the actor is stopped before the receive loop is started.

- Set the thread name of any `pykka.ThreadingActor` to `PykkaActorThread-N` instead of the default `Thread-N`. This eases debugging by clearly labeling actor threads in e.g. the output of `threading.enumerate()`.

- Add utility method `pykka.ActorRegistry.broadcast()` which broadcasts a message to all registered actors or to a given class of registered actors. (Fixes: 7)

- Allow multiple calls to `pykka.ActorRegistry.unregister()` with the same `pykka.actor.ActorRef` as argument without throwing a `ValueError`. (Fixes: 5)

- Make the `pykka.ActorProxy`'s reference to its `pykka.ActorRef` public as `pykka.ActorProxy.actor_ref`. The `ActorRef` instance was already exposed as a public field by the actor itself using the same name, but making it public directly on the proxy makes it possible to do e.g. `proxy.actor_ref.is_alive()` without waiting for a potentially dead actor to return an `ActorRef` instance you can use. (Fixes: 3)

- Actors are now registered in `pykka.registry.ActorRegistry` before they are started. This fixes a race condition where an actor tried to stop and unregister itself before it was registered, causing an exception in `ActorRegistry.unregister`.

- Stop all running actors on `BaseException` instead of just `KeyboardInterrupt`, so that `sys.exit(1)` will work.