Upgrade Akka to 2.3.0-RC1

- Upgrade Akka to 2.3.0-RC1
- Add parens to `sender` references in docs and samples as recommended
- Fix removed or deprecated Akka Java API in docs and samples
  
No further modification is needed since the basic API is not changed, except `sender()`. [The migration guide says](http://doc.akka.io/docs/akka/2.3.0-RC1/project/migration-guide-2.2.x-2.3.x.html#Parens_Added_to_sender)
:

> Parens were added to the sender() method of the Actor Scala API to highlight that the sender() reference is not referentially transparent and must not be exposed to other threads, for example by closing over it when using future callbacks.

It still just works without parens, but I'd like to follow the new convention.