+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/main/scala/akka/cluster/ddata/GSet.scala

@@ -0,0 +1,66 @@
+/**
+ * Copyright (C) 2009-2015 Typesafe Inc. <http://www.typesafe.com>
+ */
+package akka.cluster.ddata
+
+object GSet {
+  private val _empty: GSet[Any] = new GSet(Set.empty)
+  def empty[A]: GSet[A] = _empty.asInstanceOf[GSet[A]]
+  def apply(): GSet[Any] = _empty
+  /**
+   * Java API
+   */
+  def create[A](): GSet[A] = empty[A]
+
+  // unapply from case class
+}
+
+/**
+ * Implements a 'Add Set' CRDT, also called a 'G-Set'. You can't
+ * remove elements of a G-Set.
+ *
+ * It is described in the paper
+ * <a href="http://hal.upmc.fr/file/index/docid/555588/filename/techreport.pdf">A comprehensive study of Convergent and Commutative Replicated Data Types</a>.
+ *
+ * A G-Set doesn't accumulate any garbage apart from the elements themselves.
+ *
+ * This class is immutable, i.e. "modifying" methods return a new instance.
+ */
+@SerialVersionUID(1L)
+final case class GSet[A](elements: Set[A]) extends ReplicatedData with ReplicatedDataSerialization {
+
+  type T = GSet[A]
+
+  /**
+   * Java API
+   */
+  def getElements(): java.util.Set[A] = {
+    import scala.collection.JavaConverters._
+    elements.asJava
+  }
+
+  def contains(a: A): Boolean = elements(a)
+
+  def isEmpty: Boolean = elements.isEmpty
+
+  def size: Int = elements.size
+
+  /**
+   * Adds an element to the set
+   */
+  def +(element: A): GSet[A] = add(element)
+
+  /**
+   * Adds an element to the set
+   */
+  def add(element: A): GSet[A] = copy(elements + element)
+
+  override def merge(that: GSet[A]): GSet[A] = copy(elements ++ that.elements)
+}
+
+object GSetKey {
+  def create[A](id: String): Key[GSet[A]] = GSetKey(id)
+}
+
+@SerialVersionUID(1L)
+final case class GSetKey[A](_id: String) extends Key[GSet[A]](_id) with ReplicatedDataSerialization