Begin experiment with a centralized API callback registry.

The API registry generalizes the node handler system from the xep_0030
plugin so that other plugins can use it.

sleekxmpp/plugins/base.py

@@ -269,6 +269,7 @@ class BasePlugin(object):
 
     def __init__(self, xmpp, config=None):
         self.xmpp = xmpp
+        self.api = self.xmpp.api.wrap(self.name)
 
         #: A plugin's behaviour may be configurable, in which case those
         #: configuration settings will be provided as a dictionary.