Add BOSH subproject (SMACK-187)
Cleanup of build.gradle

- Moved helper code at the end
- Add description to the POM
Raise jbosh version to 0.7.0
Substitute MXParser with a call to XmlPullParserFactory

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.
Remove the dependency on javax.xml.transform

Not all platforms support this API (e.g. Android). Strip the
prettyPrint() method since it wasn't used anyway.

Also move XmlUtil in a more appropriate package.
Remove some deprecated code