+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/test/scala/akka/cluster/ddata/GSetSpec.scala

@@ -0,0 +1,119 @@
+/**
+ * Copyright (C) 2009-2015 Typesafe Inc. <http://www.typesafe.com>
+ */
+
+package akka.cluster.ddata
+
+import akka.cluster.ddata.Replicator.Changed
+import org.scalatest.Matchers
+import org.scalatest.WordSpec
+
+class GSetSpec extends WordSpec with Matchers {
+
+  val user1 = """{"username":"john","password":"coltrane"}"""
+  val user2 = """{"username":"sonny","password":"rollins"}"""
+  val user3 = """{"username":"charlie","password":"parker"}"""
+  val user4 = """{"username":"charles","password":"mingus"}"""
+
+  "A GSet" must {
+
+    "be able to add user" in {
+      val c1 = GSet.empty[String]
+
+      val c2 = c1 + user1
+      val c3 = c2 + user2
+
+      val c4 = c3 + user4
+      val c5 = c4 + user3
+
+      c5.elements should contain(user1)
+      c5.elements should contain(user2)
+      c5.elements should contain(user3)
+      c5.elements should contain(user4)
+    }
+
+    "be able to have its user set correctly merged with another GSet with unique user sets" in {
+      // set 1
+      val c11 = GSet.empty[String]
+
+      val c12 = c11 + user1
+      val c13 = c12 + user2
+
+      c13.elements should contain(user1)
+      c13.elements should contain(user2)
+
+      // set 2
+      val c21 = GSet.empty[String]
+
+      val c22 = c21 + user3
+      val c23 = c22 + user4
+
+      c23.elements should contain(user3)
+      c23.elements should contain(user4)
+
+      // merge both ways
+      val merged1 = c13 merge c23
+      merged1.elements should contain(user1)
+      merged1.elements should contain(user2)
+      merged1.elements should contain(user3)
+      merged1.elements should contain(user4)
+
+      val merged2 = c23 merge c13
+      merged2.elements should contain(user1)
+      merged2.elements should contain(user2)
+      merged2.elements should contain(user3)
+      merged2.elements should contain(user4)
+    }
+
+    "be able to have its user set correctly merged with another GSet with overlapping user sets" in {
+      // set 1
+      val c10 = GSet.empty[String]
+
+      val c11 = c10 + user1
+      val c12 = c11 + user2
+      val c13 = c12 + user3
+
+      c13.elements should contain(user1)
+      c13.elements should contain(user2)
+      c13.elements should contain(user3)
+
+      // set 2
+      val c20 = GSet.empty[String]
+
+      val c21 = c20 + user2
+      val c22 = c21 + user3
+      val c23 = c22 + user4
+
+      c23.elements should contain(user2)
+      c23.elements should contain(user3)
+      c23.elements should contain(user4)
+
+      // merge both ways
+      val merged1 = c13 merge c23
+      merged1.elements should contain(user1)
+      merged1.elements should contain(user2)
+      merged1.elements should contain(user3)
+      merged1.elements should contain(user4)
+
+      val merged2 = c23 merge c13
+      merged2.elements should contain(user1)
+      merged2.elements should contain(user2)
+      merged2.elements should contain(user3)
+      merged2.elements should contain(user4)
+    }
+
+    "have unapply extractor" in {
+      val s1 = GSet.empty + "a" + "b"
+      val s2: GSet[String] = s1
+      val GSet(elements1) = s1
+      val elements2: Set[String] = elements1
+      Changed(GSetKey[String]("key"))(s1) match {
+        case c @ Changed(GSetKey("key")) ⇒
+          val GSet(elements3) = c.dataValue
+          val elements4: Set[String] = elements3
+          elements4 should be(Set("a", "b"))
+      }
+    }
+
+  }
+}