'Add a node handler for the given hierarchy level and
handler type.
Node handlers are ordered in a hierarchy where the
most specific handler is executed. Thus, a fallback,
global handler can be used for the majority of cases
with a few node specific handler that override the
global behavior.
Node handler hierarchy:
JID   | Node  | Level
None  | None  | Global
Given | None  | All nodes for the JID
None  | Given | Node on self.xmpp.boundjid
Given | Given | A single node
Handler types:
get_info
get_items
set_identities
set_features
set_items
del_items
del_identities
del_identity
del_feature
del_features
del_item
add_identity
add_feature
add_item
Arguments:
htype   -- The operation provided by the handler.
jid     -- The JID the handler applies to. May be narrowed
further if a node is given.
node    -- The particular node the handler is for. If no JID
is given, then the self.xmpp.boundjid.full is
assumed.
handler -- The handler function to use.'

def set_node_handler(self, htype, jid=None, node=None, handler=None):

    self.api.register(handler, htype, jid, node)
