Cleanup for stanzabase.

Use stanza.values instead of _get/set_stanza_values where used.

ElementBase stanzas can now use .tag

May use class method tag_name() for stanza classes.

ElementBase now has .clear() method.

sleekxmpp/plugins/xep_0092/version.py

@@ -84,5 +84,5 @@ class xep_0092(base_plugin):
         result = iq.send()
 
         if result and result['type'] != 'error':
-            return result['software_version']._get_stanza_values()
+            return result['software_version'].values
         return False