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-tcp/src/test/java/org/jivesoftware/smack/tcp/XmppNioTcpConnectionTest.java

@@ -0,0 +1,32 @@
+/**
+ *
+ * Copyright 2018 Florian Schmaus
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.jivesoftware.smack.tcp;
+
+import org.jivesoftware.smack.fsm.StateDescriptor;
+import org.jivesoftware.smack.fsm.StateDescriptorGraph.GraphVertex;
+import org.jivesoftware.smack.tcp.XmppNioTcpConnection.InstantShutdownStateDescriptor;
+
+public class XmppNioTcpConnectionTest {
+
+    public void graphComplete() {
+        assertContains(XmppNioTcpConnection.INITIAL_STATE_DESCRIPTOR_VERTEX, InstantShutdownStateDescriptor.class);
+    }
+
+    private static void assertContains(GraphVertex<StateDescriptor> graph, Class<? extends StateDescriptor> state) {
+        throw new Error("Implement me");
+    }
+}