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-repl/build.gradle

@@ -37,3 +37,8 @@ task printClasspath(dependsOn: assemble) {
         println sourceSets.main.runtimeClasspath.asPath
     }
 }
+
+task printXmppNioTcpConnectionStateGraph(type: JavaExec) {
+	classpath sourceSets.main.runtimeClasspath
+	main 'org.igniterealtime.smack.smackrepl.StateGraph'
+}