=pro,doc Update sbt-multi-jvm plugin to 0.3.8 and add sbt 0.13 docs
!act #3581 Add Tcp.CompoundWrite, some cleanup

Moves `def ack: Event` and `def wantsAck: Boolean` from the `Tcp.WriteCommand` type down to the newly introduced `Tcp.CompactWriteCommand`, which breaks existing code depending on these.
Additionally `Tcp.WriteCommand` now has a few additional methods (`+:`, `++:`, prepend) and a companion object.
=doc #3581 Add "Writing to a connection" section to IO docs
Merge pull request #1709 from spray/wip-3581-io-compound-write

!act #3581 Add Tcp.CompoundWrite, some cleanup
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)
Merge pull request #1751 from eligosource/wip-3641-storage-plugin-apis-krasserm

+per #3641 Storage plugin API