Add support for Nicks
Nick elements aren't properly parsed by the DefaultPacketExtension. We should add a new PacketExtension for nickname extensions.
The Nick class should be moved into a package called *.smackx.nick. We want to standardize how the extensions are represented in Smack in this fashion. Otherwise (as is currently the case with many extensions) all classes simply go in a couple of high level packages, which quickly becomes unwieldy as new extensions and classes are added.