=actor: More descriptive error message when serializer with given ID is missing #3487
Handle null constructor parameters, see #3486
DOC: how to change default mailbox, see #3483
DOC: FSM start state in preStart/postRestart, see #3440
Adjust dist README, see #3105
Fix scaladoc warnings in akka-actor, see #3152

* also change wrong visibility of SystemMessage subclasses
Improve scaladoc in akka-actor, see #3152

* also change visibility of Actor.context