Add XmlStringBuilder.emptyElement(Enum)
Use XmlStringBuilder in AdHocCommandData

also remove 'lang' attribute, since it belongs into IQ, see XEP-50 3.7:

"The requester SHOULD provide its locale information using the "xml:lang"
attribute on either the <iq/> (RECOMMENDED) or <command/> element."
Remove XmlLangStanza

since all stanza types could have 'xml:lang' defined as top-level
stream element attribute.
Add ELEMENT and NAMESPACE to RosterPacket
Add support for (custom) data form extensions

Fixes SMACK-605
Remove smack.util.Cache

and use the Cache from jxmpp-util-cache instead.
Support for International Domain Names in DNSUtil