+cdd #16799 Add Distributed Data module

Previously know as [patriknw/akka-data-replication](https://github.com/patriknw/akka-data-replication),
which was originally inspired by [jboner/akka-crdt](https://github.com/jboner/akka-crdt).

The functionality is very similar to akka-data-replication 0.11.

Here is a list of the most important changes:

* The package name changed to `akka.cluster.ddata`
* The extension was renamed to `DistributedData`
* The keys changed from strings to classes with unique identifiers and type information of the data values,
  e.g. `ORSetKey[Int]("set2")`
* The optional read consistency parameter was removed from the `Update` message. If you need to read from
  other replicas before performing the update you have to first send a `Get` message and then continue with
  the ``Update`` when the ``GetSuccess`` is received.
* `BigInt` is used in `GCounter` and `PNCounter` instead of `Long`
* Improvements of java api
* Better documentation

akka-distributed-data/src/multi-jvm/scala/akka/cluster/ddata/STMultiNodeSpec.scala

@@ -0,0 +1,20 @@
+/**
+ * Copyright (C) 2009-2015 Typesafe Inc. <http://www.typesafe.com>
+ */
+package akka.cluster.ddata
+
+import akka.remote.testkit.MultiNodeSpecCallbacks
+
+import org.scalatest.{ BeforeAndAfterAll, WordSpecLike }
+import org.scalatest.Matchers
+
+/**
+ * Hooks up MultiNodeSpec with ScalaTest
+ */
+trait STMultiNodeSpec extends MultiNodeSpecCallbacks
+  with WordSpecLike with Matchers with BeforeAndAfterAll {
+
+  override def beforeAll() = multiNodeSpecBeforeAll()
+
+  override def afterAll() = multiNodeSpecAfterAll()
+}