This makes Smack more portable, as there are platforms that support the
XmlPullParser interface, but not MXParser (e.g. Android).
Also enable checkstyle check that MXParser is not used.
The jingle subproject builds now. This doesn't change that the code is
outdated with regard to the specification and unmaintained for
years. But hopefully this is the first step to change that. :)
The integration tests have been moved into SourceSets of 'core' and
'extensions'.
