API adjustments to ClusterClient, see #3264

* Message wrappers in ClusterClient, so that DistributedPubSubMediator
  is not leaking to the client api
* Register methods in ClusterReceptionistExtension, for convenience and
  clarity
Use ScalaBuff 1.2.0-SNAPSHOT so the build works on windows.
Merge pull request #1359 from akka/wip-cluster-client-api

API adjustments to ClusterClient, see #3264
Merge pull request #1366 from akka/wip-use-scalabuff-snapshot-so-build-works-on-windows-ban

Use ScalaBuff 1.2.0-SNAPSHOT so the build works on windows.
Move resolver so that all dependent projects find the artifact.