SmackReactor/NIO, Java8/Android19, Pretty print XML, FSM connections

This commit adds
- SmackReactor / NIO
- a framework for finite state machine connections
- support for Java 8
- pretty printed XML debug output

It also
- reworks the integration test framework
- raises the minimum Android API level to 19
- introduces XmppNioTcpConnection

Furthermore fixes SMACK-801 (at least partly). Java 8 language
features are available, but not all runtime library methods. For that
we would need to raise the Android API level to 24 or higher.

smack-integration-test/src/main/java/org/jivesoftware/smack/ChatTest.java

@@ -1,6 +1,6 @@
 /**
  *
- * Copyright 2015 Florian Schmaus
+ * Copyright 2015-2019 Florian Schmaus
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -48,7 +48,7 @@ public class ChatTest extends AbstractSmackIntegrationTest {
     private boolean invoked;
 
     @SuppressWarnings("deprecation")
-    public ChatTest(SmackIntegrationTestEnvironment environment) {
+    public ChatTest(SmackIntegrationTestEnvironment<?> environment) {
         super(environment);
         chatManagerOne = org.jivesoftware.smack.chat.ChatManager.getInstanceFor(conOne);
     }