core (xmpp): server certificate validation:

XMPP server certificate is now checked, and connection is refused (by default) if it's not valid.
Certificate check can be disabled in the new parameter "Configuration/check_certificate".
If certificate checking is disabled, a warning note is sent on every new connection.
Twisted and Wokkel are temporarly monkey patched in sat.core.tls_patches module, until modifications are merged upstream.

sat/core/exceptions.py

@@ -121,3 +121,8 @@ class ParsingError(Exception):
 # Something which need to be done is not available yet
 class NotReady(Exception):
     pass
+
+
+class InvalidCertificate(Exception):
+    """A TLS certificate is not valid"""
+    pass