+per #17579 #17617 Introduces EventAdapter

+ per plugin scoped adapters
+ could be swapped during runtime
+per EventAdapter now has manifest and is configurable ai la serializers
+ json examples in docs
+ including "completely manual" example in case one wants to add
  metadata TO the persisted event
+ better error reporting when misconfigured bindings
+ manifest is handled by in memory plugin
- did not check if it works with LevelDB plugin yet
> TODO: json example uses Gson, as that's simplest to do, can we use
+per allows 1:n adapters, multiple adapters can be bound to 1 class

akka-docs/rst/java/code/docs/persistence/PersistenceEventAdapterDocTest.java

@@ -0,0 +1,32 @@
+/*
+ * Copyright (C) 2015 Typesafe Inc. <http://www.typesafe.com>
+ */
+
+package docs.persistence;
+
+import akka.persistence.journal.EventAdapter;
+import akka.persistence.journal.EventSeq;
+
+public class PersistenceEventAdapterDocTest {
+
+  @SuppressWarnings("unused")
+  static
+  //#identity-event-adapter
+  class MyEventAdapter extends EventAdapter {
+    @Override
+    public String manifest(Object event) {
+      return ""; // if no manifest needed, return ""
+    }
+
+    @Override
+    public Object toJournal(Object event) {
+      return event; // identity
+    }
+
+    @Override
+    public EventSeq fromJournal(Object event, String manifest) {
+      return EventSeq.single(event); // identity
+    }
+  }
+  //#identity-event-adapter
+}