Fix "Could not initialize class SmackConfiguration"

Fixes #1804 and #1718 (to some amount). The exception ("Could not initialize ...") was thrown because no XmlPullParser was found in the classpath. While developing for openhab, xpp3 slipped into the classpath but wasn't committed. Sorry for that.

Anyway, I'd like to develop further on the xmpp action. Since Smack now enforces SSL/TLS security, I'd like to add support for [java pinning](https://github.com/flowdalic/java-pinning) so that users can enjoy *secure* TLS connections, even when the server certificate is not signed by a CA cert found in the default SSLContext. Should I base my work on the '1.6.1' or 'master' branch?