Add support for X-GOOGLE-TOKEN.

This is mainly just useful for authenticating without using TLS.

If an access token is not provided, an attempt will be made to
retrieve one from Google.

sleekxmpp/thirdparty/suelta/mechanisms/plain.py

@@ -58,4 +58,4 @@ class PLAIN(Mechanism):
         return True
 
 
-register_mechanism('PLAIN', 1, PLAIN, use_hashes=False)
+register_mechanism('PLAIN', 5, PLAIN, use_hashes=False)