Added session_end event and some docs.

For now, session_end is the same as disconnected, but once support is
added later for stream management, the two events will become distinct.

Plugins should add handlers for session_end for cleaning any session
state.

sleekxmpp/xmlstream/xmlstream.py

@@ -389,6 +389,7 @@ class XMLStream(object):
             self.event('socket_error', serr)
         finally:
             #clear your application state
+            self.event('session_end', direct=True)
             self.event("disconnected", direct=True)
             return True