Export list is specified.
Added reply on ping (XEP-0199).

Stanza result sending function is extended by the "to" param.
The handle_notification function is changed so it could handle some
notification by itself (like this ping) rather then just reading and
returning result.
The handle_notification is renamed to handle_info.
Added ability to build error stanzas.
Added function for sending error stanzas.
Added handling of the unsupported get stanzas.
Added request vcard function.