and not NullPointerException. Altough this differs from
java.util.Objects behavior, throwing an IllegalArgumentException
appears more sensible and makes it easier to catch it in Smack's
parsing function.
Introducing Smack's own XmlPullParser interface which tries to stay as
compatible as possible to XPP3. The interface is used to either wrap
StAX's XMLStreamReader if Smack is used on Java SE, and XPP3's
XmlPullParser if Smack is used on on Android.
Fixes SMACK-591.
Also introduce JUnit 5 and non-strict javadoc projects.
