CopyWithZone within the XMPPPresence

Hi,

Here is a little fix for a bug that has been annoying for me:
The XMPPPresence didn't implement the copyWithZone. So, the returned object of the 'copy' method was a XMPPElement which thrown 'unrecognized selector' exceptions on the XMPPPresence methods.

Cheers,
Romain