Fix a bug. Adding the copyWithZone within the XMPPPresence. Now the copied object is a XMPPPresence instance.
copyWithZone added to XMPPIQ, XMPPMessage and XMPPPresence

Fixes #229
xmppRoster:didReceiveRosterPush: API

Sent when a Roster Push is received as specified in Section 2.1.6 of RFC 6121.
Initialize Variable

Fixes #231
registerWithElements:error API

Fixes #232