=pro,doc Update sbt-multi-jvm plugin to 0.3.8 and add sbt 0.13 docs
Merge pull request #1743 from akka/wip-upgrade-sbt-multijvm-ban

=pro,doc Update sbt-multi-jvm plugin to 0.3.8 and add sbt 0.13 docs
+per #3641 Storage plugin API

- Journal plugin API for storage backends with asynchronous client API (default impl: in-memory journal)
- Journal plugin API for storage backends with synchronous client API (default impl: LevelDB journal)
- Snapshot store plugin API (default impl: local filesystem snapshot store)