Also change StringUtils.escapeForXML() and Packet.toXML() to return
CharSequence instead of String. XmlStringBuilder now has 'optX' methods.
Remove XmlUtils in favor of XmlStringBuilder
If whitespace (e.g. \n \t) needs to be preserved, then XML elements
should be used instead of attributes. Since we do this now in
DirectoryRosterStore, there is also no longer a need for an
specialized StringUtils.escapeForXML().
Also introduce XmlStringBuilder, which should become the default way
to implement Packet.toXML() and the like.