Add OMEMO support

This commit adds the modules smack-omemo and smack-omemo-signal.
smack-omemo is licensed under the Apache license like the rest of the smack project.
smack-omemo-signal on the other hand is licensed under the GPLv3.
Due to the fact, that smack-omemo is not of much use without smack-omemo-signal,
the OMEMO feature can currently only be used by GPLv3 compatible software.
This may change in the future, when a more permissively licensed module becomes available.

Fixes SMACK-743.

smack-omemo-signal-integration-test/build.gradle

@@ -0,0 +1,17 @@
+apply plugin: 'application'
+
+description = """\
+Smack integration tests for OMEMO using libsignal."""
+
+mainClassName = 'org.igniterealtime.smack.inttest.smack_omemo_signal.SmackOmemoSignalIntegrationTestFramework'
+applicationDefaultJvmArgs = ["-enableassertions"]
+
+dependencies {
+	compile project(':smack-integration-test')
+	compile project(':smack-omemo-signal')
+}
+
+run {
+	// Pass all system properties down to the "application" run
+	systemProperties System.getProperties()
+}