Add has_item variants checking for other attrs

An <item/> element must contain a <<"jid">> attr
and may contain a <<"node">> and a <<"name">> attr.
Add <reach/> element helper
Add a number of pubsub stanza constructors
Add iq/4 stanza constructor
Remove the non-obligatory 'from' attribute
Add a publish/3 which allows for specifying the recipient
Add create_node/2 pubsub stanza
Add subscribe/3 pubsub stanza
Reorder the stanzas to match the order in -export()
Add is_node_created/2 predicate
Add is_pubsub_event/1 predicate
Add is_subscription/3 predicate
Add is_publish_result/2 predicate
Add is_disco_info/1 predicate