Move extension relevant configuration options from SmackConfiguration to
the extension. Introduced disabledSmackClasses that can be configured
via a system property or configuration file.
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.
Also move ProviderConfigTest into core, since it tests core
functionality, nothing provided by extensions. Found the reason the test
was failing since the gradle migration (provider entry test.providers),
and activated it again. \o/
New API design as of SMACK-545
