Add support for MSN with X-MESSENGER-OAUTH2 SASL support.

NOTE: This requires already having the access token. It does NOT
perform any OAuth requests.

sleekxmpp/features/feature_mechanisms/stanza/auth.py

@@ -25,15 +25,25 @@ class Auth(StanzaBase):
     interfaces = set(('mechanism', 'value'))
     plugin_attrib = name
 
+    #: Some SASL mechs require sending values as is, 
+    #: without converting base64.
+    plain_mechs = set(['X-MESSENGER-OAUTH2'])
+
     def setup(self, xml):
         StanzaBase.setup(self, xml)
         self.xml.tag = self.tag_name()
 
     def get_value(self):
-        return base64.b64decode(bytes(self.xml.text))
+        if not self['mechanism'] in self.plain_mechs:
+            return base64.b64decode(bytes(self.xml.text))
+        else:
+            return self.xml.text
 
     def set_value(self, values):
-        self.xml.text = bytes(base64.b64encode(values)).decode('utf-8')
+        if not self['mechanism'] in self.plain_mechs:
+            self.xml.text = bytes(base64.b64encode(values)).decode('utf-8')
+        else:
+            self.xml.text = values
 
     def del_value(self):
         self.xml.text = ''