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-resolver-minidns/src/main/java/org/jivesoftware/smack/util/dns/minidns/MiniDnsDaneVerifier.java

@@ -23,6 +23,7 @@ import java.util.logging.Logger;
 
 import javax.net.ssl.KeyManager;
 import javax.net.ssl.SSLContext;
+import javax.net.ssl.SSLSession;
 import javax.net.ssl.SSLSocket;
 import javax.net.ssl.TrustManager;
 import javax.net.ssl.X509TrustManager;
@@ -68,4 +69,17 @@ public class MiniDnsDaneVerifier implements SmackDaneVerifier {
         }
     }
 
+    @Override
+    public void finish(SSLSession sslSession) throws CertificateException {
+        if (VERIFIER.verify(sslSession)) {
+            // DANE verification was the only requirement according to the TLSA RR. We can return here.
+            return;
+        }
+
+        // DANE verification was successful, but according to the TLSA RR we also must perform PKIX validation.
+        if (expectingTrustManager.hasException()) {
+         // PKIX validation has failed. Throw an exception.
+            throw expectingTrustManager.getException();
+        }
+    }
 }