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.
The initializer tests verify that every non-optional initializer, this
includes Providers, is loadable.
Creating files under META-INF is not considered best practice. Smack's
configuration and provider files reside now in classpath directory
qualified by Smack's package namespace.
