+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/src/main/protobuf/TwoPhaseSetMessages.proto

@@ -0,0 +1,24 @@
+/**
+ * Copyright (C) 2014-2015 Typesafe Inc. <http://www.typesafe.com>
+ */
+package docs.ddata;
+
+//#twophaseset
+option java_package = "docs.ddata.protobuf.msg";
+option optimize_for = SPEED;
+
+message TwoPhaseSet {
+  repeated string adds = 1;
+  repeated string removals = 2;
+}
+//#twophaseset
+
+//#twophaseset2
+message TwoPhaseSet2 {
+  optional bytes adds = 1;
+  optional bytes removals = 2;
+}
+//#twophaseset2
+
+
+