+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-docs/rst/scala/code/docs/ddata/TwoPhaseSet.scala

@@ -0,0 +1,29 @@
+/**
+ * Copyright (C) 2015 Typesafe Inc. <http://www.typesafe.com>
+ */
+package docs.ddata
+
+import akka.cluster.ddata.ReplicatedData
+import akka.cluster.ddata.GSet
+
+//#twophaseset
+case class TwoPhaseSet(
+  adds: GSet[String] = GSet.empty,
+  removals: GSet[String] = GSet.empty)
+  extends ReplicatedData {
+  type T = TwoPhaseSet
+
+  def add(element: String): TwoPhaseSet =
+    copy(adds = adds.add(element))
+
+  def remove(element: String): TwoPhaseSet =
+    copy(removals = removals.add(element))
+
+  def elements: Set[String] = adds.elements -- removals.elements
+
+  override def merge(that: TwoPhaseSet): TwoPhaseSet =
+    copy(
+      adds = GSet(this.adds.elements ++ that.adds.elements),
+      removals = GSet(this.removals.elements ++ that.removals.elements))
+}
+//#twophaseset